Handle nil pointer in Lb delete

[shyamradhakrishnan]

What this PR does / why we need it:

Handle nil pointer in Lb backend delete. This happens when instance was created, but was not added to backend because the instance does not have IP address. Then when instance is deleted, we get the following stack trace.

panic: runtime error: index out of range [0] with length 0 [recovered]
        panic: runtime error: index out of range [0] with length 0

goroutine 553 [running]:
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Reconcile.func1()
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:118 +0x1f4
panic({0x1dd91a0, 0xc0001c0b10})
        /usr/local/go/src/runtime/panic.go:884 +0x212
github.com/oracle/cluster-api-provider-oci/cloud/scope.(*MachineScope).IP(...)
        /workspace/cloud/scope/machine.go:402
github.com/oracle/cluster-api-provider-oci/cloud/scope.(*MachineScope).ReconcileDeleteInstanceOnLB(0xc000a02280, {0x2236998, 0xc000ce69c0})
        /workspace/cloud/scope/machine.go:621 +0xf49
github.com/oracle/cluster-api-provider-oci/controllers.(*OCIMachineReconciler).deleteInstanceFromControlPlaneLB(0xc0003b4ff0, {0x2236998, 0xc000ce69c0}, 0xc000a02280)
        /workspace/controllers/ocimachine_controller.go:376 +0x85
github.com/oracle/cluster-api-provider-oci/controllers.(*OCIMachineReconciler).reconcileDelete(0xc0003b4ff0, {0x2236998, 0xc000ce69c0}, 0xc000a02280)
        /workspace/controllers/ocimachine_controller.go:324 +0x54d
github.com/oracle/cluster-api-provider-oci/controllers.(*OCIMachineReconciler).Reconcile(0xc0003b4ff0, {0x2236998, 0xc000ce69c0}, {{{0xc0007b2020?, 0x10?}, {0xc000f19d00?, 0x40db67?}}})
        /workspace/controllers/ocimachine_controller.go:161 +0xada
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Reconcile(0x22368f0?, {0x2236998?, 0xc000ce69c0?}, {{{0xc0007b2020?, 0x1e40220?}, {0xc000f19d00?, 0x4045d4?}}})
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:121 +0xc8
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0xc000196820, {0x22368f0, 0xc00020b500}, {0x1cdf440?, 0xc0001bb340?})
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:320 +0x33c
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem(0xc000196820, {0x22368f0, 0xc00020b500})
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:273 +0x1d9
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2.2()
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:234 +0x85
created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2
        /workspace/vendor/sigs.k8s.io/controller-runtime/pkg/internal/controller/controller.go:230 +0x333

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #275

[shyamradhakrishnan]

e2e

Ran 6 of 25 Specs in 2610.524 seconds
SUCCESS! -- 6 Passed | 0 Failed | 0 Pending | 19 Skipped

[shyamradhakrishnan]

unit test

?   	github.com/oracle/cluster-api-provider-oci	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/api/v1beta1	41.273s	coverage: 24.1% of statements
ok  	github.com/oracle/cluster-api-provider-oci/api/v1beta2	0.043s	coverage: 15.8% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/config	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/metrics	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/cloud/ociutil	0.004s	coverage: 19.6% of statements
ok  	github.com/oracle/cluster-api-provider-oci/cloud/scope	241.278s	coverage: 76.3% of statements
?   	github.com/oracle/cluster-api-provider-oci/cloud/scope/mocks	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/base	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/base/mock_base	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/compute	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/compute/mock_compute	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/computemanagement	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/computemanagement/mock_computemanagement	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/containerengine	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/containerengine/mock_containerengine	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/identity	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/identity/mock_identity	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/loadbalancer	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/loadbalancer/mock_lb	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer/mock_nlb	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/vcn	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/cloud/services/vcn/mock_vcn	[no test files]
ok  	github.com/oracle/cluster-api-provider-oci/cloud/util	0.128s	coverage: 76.1% of statements
ok  	github.com/oracle/cluster-api-provider-oci/controllers	26.128s	coverage: 67.5% of statements
ok  	github.com/oracle/cluster-api-provider-oci/exp/api/v1beta1	27.261s	coverage: 21.8% of statements
ok  	github.com/oracle/cluster-api-provider-oci/exp/api/v1beta2	0.023s	coverage: 10.9% of statements
ok  	github.com/oracle/cluster-api-provider-oci/exp/controllers	1.095s	coverage: 56.3% of statements
?   	github.com/oracle/cluster-api-provider-oci/feature	[no test files]
?   	github.com/oracle/cluster-api-provider-oci/version	[no test files]