Introduced in CDRIVER-3423 in this commit:
https://github.com/mongodb/mongo-c-driver/commit/535e9091f399fbea2cacab490b93fbed48c328be#diff-3d3e42655bbb89f4b06256d8b8480e3eR369

If the environment variable AWS_CONTAINER_CREDENTIALS_RELATIVE_URI is set, but is the empty string, this function leaks that empty string.