data-source/aws_ecr_image: Use AWS Go SDK provided conversion functions to prevent panics
data-source/aws_ecr_image: Use AWS Go SDK provided conversion functions to prevent panics

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/8403

Output from acceptance testing:

```
--- PASS: TestAccAWSEcrDataSource_ecrImage (14.26s)
--- PASS: TestAccAWSEcrDataSource_ecrRepository (17.23s)
```

Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws