In kms-message, calling kms_request_get_signed without setting AWS credentials should fail but doesn't. kms-message doesn't error but gives you a malformed request. We should set an error on the passed kms_request_t (using KMS_ERROR) and return NULL. Let's add a test in test_kms_request.c to verify the behavior.

It may be worthwhile to have other functions in kms-message fail as well.