provider/aws: Safely get ELB values

[catsby]

Removes dereferences to avoid possible nil pointer dereferences crashes; d.Set can safely handle nil values.

Fixes #7555

[catsby]

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSELB_ -timeout 120m
=== RUN   TestAccAWSELB_importBasic
--- PASS: TestAccAWSELB_importBasic (24.12s)
=== RUN   TestAccAWSELB_basic
--- PASS: TestAccAWSELB_basic (21.92s)
=== RUN   TestAccAWSELB_fullCharacterRange
--- PASS: TestAccAWSELB_fullCharacterRange (20.90s)
=== RUN   TestAccAWSELB_AccessLogs
--- PASS: TestAccAWSELB_AccessLogs (49.66s)
=== RUN   TestAccAWSELB_generatedName
--- PASS: TestAccAWSELB_generatedName (18.90s)
=== RUN   TestAccAWSELB_availabilityZones
--- PASS: TestAccAWSELB_availabilityZones (40.25s)
=== RUN   TestAccAWSELB_tags
--- PASS: TestAccAWSELB_tags (33.27s)
=== RUN   TestAccAWSELB_iam_server_cert
--- PASS: TestAccAWSELB_iam_server_cert (39.06s)
=== RUN   TestAccAWSELB_InstanceAttaching
--- PASS: TestAccAWSELB_InstanceAttaching (113.72s)
=== RUN   TestAccAWSELB_HealthCheck
--- PASS: TestAccAWSELB_HealthCheck (17.98s)
=== RUN   TestAccAWSELB_Timeout
--- PASS: TestAccAWSELB_Timeout (16.05s)
=== RUN   TestAccAWSELB_ConnectionDraining
--- PASS: TestAccAWSELB_ConnectionDraining (17.44s)
=== RUN   TestAccAWSELB_SecurityGroups
--- PASS: TestAccAWSELB_SecurityGroups (31.70s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    444.979s
Test:

[stack72]

LGTM!

[ghost]

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