r/aws_eip_association: Avoid crash in EC2 Classic

[radeksimko]

Fixes #337

Test results

make testacc TEST=./aws TESTARGS='-run=TestAccAWSEIP'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSEIP -timeout 120m
=== RUN   TestAccAWSEIPAssociation_basic
--- PASS: TestAccAWSEIPAssociation_basic (159.44s)
=== RUN   TestAccAWSEIPAssociation_ec2Classic
--- PASS: TestAccAWSEIPAssociation_ec2Classic (207.69s)
=== RUN   TestAccAWSEIPAssociation_disappears
--- PASS: TestAccAWSEIPAssociation_disappears (147.68s)
=== RUN   TestAccAWSEIP_importEc2Classic
--- PASS: TestAccAWSEIP_importEc2Classic (141.03s)
=== RUN   TestAccAWSEIP_importVpc
--- PASS: TestAccAWSEIP_importVpc (94.51s)
=== RUN   TestAccAWSEIP_basic
--- PASS: TestAccAWSEIP_basic (37.61s)
=== RUN   TestAccAWSEIP_instance
--- PASS: TestAccAWSEIP_instance (234.50s)
=== RUN   TestAccAWSEIP_network_interface
--- PASS: TestAccAWSEIP_network_interface (93.46s)
=== RUN   TestAccAWSEIP_twoEIPsOneNetworkInterface
--- PASS: TestAccAWSEIP_twoEIPsOneNetworkInterface (89.93s)
=== RUN   TestAccAWSEIP_associated_user_private_ip
--- PASS: TestAccAWSEIP_associated_user_private_ip (236.41s)
=== RUN   TestAccAWSEIP_classic_disassociate
--- PASS: TestAccAWSEIP_classic_disassociate (300.25s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	1742.539s

[catsby]

👍

[ghost]

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