tests/resource/aws_network_acl: Prevent crash in TestAccAWSNetworkAcl_SubnetChange

[bflad]

Old and busted (occasionally):

=== RUN   TestAccAWSNetworkAcl_SubnetChange
--- FAIL: TestAccAWSNetworkAcl_SubnetChange (25.21s)

------- Stderr: -------
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x2507806]

goroutine 22 [running]:
testing.tRunner.func1(0xc4202bd3b0)
    /usr/local/go/src/testing/testing.go:711 +0x2d2
panic(0x2b65040, 0x5218270)
    /usr/local/go/src/runtime/panic.go:491 +0x283
github.com/terraform-providers/terraform-provider-aws/aws.testAccCheckSubnetIsAssociatedWithAcl.func1(0xc4208a42a0, 0xbbc15f, 0xc42094b620)
    /opt/teamcity-agent/work/6025502d125bbe5e/src/github.com/terraform-providers/terraform-provider-aws/aws/resource_aws_network_acl_test.go:506 +0x136

New hotness:

make testacc TEST=./aws TESTARGS='-run=TestAccAWSNetworkAcl_SubnetChange'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSNetworkAcl_SubnetChange -timeout 120m
=== RUN   TestAccAWSNetworkAcl_SubnetChange
--- PASS: TestAccAWSNetworkAcl_SubnetChange (53.40s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	53.441s

[tombuildsstuff]

LGTM 👍

[ghost]

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