resource/aws_rds_cluster_instance: Prevent crash on importing non-cluster instances

[bflad]

Reference: #3959

I'm not sure its possible to test importing the wrong resource type in the provider acceptance testing framework, but this nil check is to prevent:

panic: runtime error: invalid memory address or nil pointer dereference
2018-03-28T21:54:57.071+0300 [DEBUG] plugin.terraform-provider-aws_v1.12.0_x4: [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x29b326b]
2018-03-28T21:54:57.071+0300 [DEBUG] plugin.terraform-provider-aws_v1.12.0_x4:
2018-03-28T21:54:57.071+0300 [DEBUG] plugin.terraform-provider-aws_v1.12.0_x4: goroutine 105 [running]:
2018-03-28T21:54:57.071+0300 [DEBUG] plugin.terraform-provider-aws_v1.12.0_x4: github.com/terraform-providers/terraform-provider-aws/aws.resourceAwsRDSClusterInstanceRead(0xc42030e3f0, 0x324f740, 0xc420095180, 0xc42030e3f0, 0x0)
2018-03-28T21:54:57.071+0300 [DEBUG] plugin.terraform-provider-aws_v1.12.0_x4: 	/opt/teamcity-agent/work/222ea50a1b4f75f4/src/github.com/terraform-providers/terraform-provider-aws/aws/resource_aws_rds_cluster_instance.go:319 +0x17b

[mbfrahry]

LGTM

[bflad]

This has been released in version 1.14.0 of the AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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