provider/aws: Remove unsafe ptr dereferencing from ECS/ECR

[radeksimko]

See full explanation & motivation in #8512 (comment)

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=AWSEcs'

TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSEcs -timeout 120m
=== RUN   TestAccAWSEcsDataSource_ecsContainerDefinition
--- PASS: TestAccAWSEcsDataSource_ecsContainerDefinition (109.07s)
=== RUN   TestAccAWSEcsCluster_basic
--- PASS: TestAccAWSEcsCluster_basic (16.14s)
=== RUN   TestAccAWSEcsServiceWithARN
--- PASS: TestAccAWSEcsServiceWithARN (120.29s)
=== RUN   TestAccAWSEcsServiceWithFamilyAndRevision
--- PASS: TestAccAWSEcsServiceWithFamilyAndRevision (109.85s)
=== RUN   TestAccAWSEcsServiceWithRenamedCluster
--- PASS: TestAccAWSEcsServiceWithRenamedCluster (232.85s)
=== RUN   TestAccAWSEcsService_withIamRole
--- PASS: TestAccAWSEcsService_withIamRole (124.40s)
=== RUN   TestAccAWSEcsService_withDeploymentValues
--- PASS: TestAccAWSEcsService_withDeploymentValues (117.27s)
=== RUN   TestAccAWSEcsService_withLbChanges
--- PASS: TestAccAWSEcsService_withLbChanges (234.16s)
=== RUN   TestAccAWSEcsService_withEcsClusterName
--- PASS: TestAccAWSEcsService_withEcsClusterName (119.98s)
=== RUN   TestAccAWSEcsService_withAlb
--- PASS: TestAccAWSEcsService_withAlb (129.50s)
=== RUN   TestAccAWSEcsTaskDefinition_basic
--- PASS: TestAccAWSEcsTaskDefinition_basic (27.26s)
=== RUN   TestAccAWSEcsTaskDefinition_withScratchVolume
--- PASS: TestAccAWSEcsTaskDefinition_withScratchVolume (16.08s)
=== RUN   TestAccAWSEcsTaskDefinition_withEcsService
--- PASS: TestAccAWSEcsTaskDefinition_withEcsService (149.09s)
=== RUN   TestAccAWSEcsTaskDefinition_withTaskRoleArn
--- PASS: TestAccAWSEcsTaskDefinition_withTaskRoleArn (18.15s)
=== RUN   TestAccAWSEcsTaskDefinition_withNetworkMode
--- PASS: TestAccAWSEcsTaskDefinition_withNetworkMode (17.81s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    1541.928s

make testacc TEST=./builtin/providers/aws TESTARGS='-run=AWSEcr'

TF_ACC=1 go test ./builtin/providers/aws -v -run=AWSEcr -timeout 120m
=== RUN   TestAccAWSEcrRepository_importBasic
--- PASS: TestAccAWSEcrRepository_importBasic (37.27s)
=== RUN   TestAccAWSEcrRepositoryPolicy_basic
--- PASS: TestAccAWSEcrRepositoryPolicy_basic (34.62s)
=== RUN   TestAccAWSEcrRepository_basic
--- PASS: TestAccAWSEcrRepository_basic (37.74s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    109.648s

[stack72]

LGTM!

[ghost]

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