Fix Cognito Userpool email_subject_by_link

[mil]

Currently creating an aws_cognito_userpool with an email_message_by_link set with an email_subject_by_link unset will segfault terraform. This commit prevents that segfault.

@Ninir, looks like a simple typo in the work recently merged around #232.

[Ninir]

Hi @mil

Thanks for the catch: this seems to have been underlooked... thanks for the fix! 👍

$ make testacc TEST=./aws TESTARGS='-run=TestAccAWSCognitoUserPool_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccAWSCognitoUserPool_ -timeout 120m
=== RUN   TestAccAWSCognitoUserPool_importBasic
--- PASS: TestAccAWSCognitoUserPool_importBasic (16.95s)
=== RUN   TestAccAWSCognitoUserPool_basic
--- PASS: TestAccAWSCognitoUserPool_basic (16.69s)
=== RUN   TestAccAWSCognitoUserPool_withAdminCreateUserConfiguration
--- PASS: TestAccAWSCognitoUserPool_withAdminCreateUserConfiguration (25.20s)
=== RUN   TestAccAWSCognitoUserPool_withDeviceConfiguration
--- PASS: TestAccAWSCognitoUserPool_withDeviceConfiguration (26.43s)
=== RUN   TestAccAWSCognitoUserPool_withEmailVerificationMessage
--- PASS: TestAccAWSCognitoUserPool_withEmailVerificationMessage (24.82s)
=== RUN   TestAccAWSCognitoUserPool_withSmsVerificationMessage
--- PASS: TestAccAWSCognitoUserPool_withSmsVerificationMessage (25.93s)
=== RUN   TestAccAWSCognitoUserPool_withEmailConfiguration
--- PASS: TestAccAWSCognitoUserPool_withEmailConfiguration (26.19s)
=== RUN   TestAccAWSCognitoUserPool_withSmsConfiguration
--- PASS: TestAccAWSCognitoUserPool_withSmsConfiguration (33.19s)
=== RUN   TestAccAWSCognitoUserPool_withSmsConfigurationUpdated
--- PASS: TestAccAWSCognitoUserPool_withSmsConfigurationUpdated (45.02s)
=== RUN   TestAccAWSCognitoUserPool_withTags
--- PASS: TestAccAWSCognitoUserPool_withTags (26.27s)
=== RUN   TestAccAWSCognitoUserPool_withAliasAttributes
--- PASS: TestAccAWSCognitoUserPool_withAliasAttributes (26.73s)
=== RUN   TestAccAWSCognitoUserPool_withPasswordPolicy
--- PASS: TestAccAWSCognitoUserPool_withPasswordPolicy (24.86s)
=== RUN   TestAccAWSCognitoUserPool_withLambdaConfig
--- PASS: TestAccAWSCognitoUserPool_withLambdaConfig (47.65s)
=== RUN   TestAccAWSCognitoUserPool_withSchemaAttributes
--- PASS: TestAccAWSCognitoUserPool_withSchemaAttributes (24.83s)
=== RUN   TestAccAWSCognitoUserPool_withVerificationMessageTemplate
--- PASS: TestAccAWSCognitoUserPool_withVerificationMessageTemplate (29.46s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	420.238s

[ghost]

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