Avoid nullptr dereference in sts creds provider client

[SergeyRyabinin]

Issue #, if available:
Subj, avoid nullptr dereference
Description of changes:
Check for ptr being non-null, don't call

user_data->callback(user_data->credentials, user_data->error_code, user_data->user_data);

(as already called in s_clean_up_user_data).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[bretambrose]

Seems fine but technically we would have crashed earlier because we're dereferencing provider_user_data from the very first line in the function.

[SergeyRyabinin]

Seems fine but technically we would have crashed earlier because we're dereferencing provider_user_data from the very first line in the function.

thanks for the comment!

The original finding was

    if (provider_user_data) {
        s_clean_up_user_data(provider_user_data);
    } else {
        provider_user_data->callback(NULL, provider_user_data->error_code, provider_user_data->user_data);
    }

where the ptr provider_user_data is first checked and in case of NULL is being immediately dereferenced to call a callback, which is obviously wrong.
Another option is to simply remove the nullptr check.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.55%. Comparing base (277c8fb) to head (8625bca).

Files	Patch %	Lines
source/credentials_provider_sts.c	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #173   +/-   ##
=======================================
  Coverage   79.54%   79.55%           
=======================================
  Files          33       33           
  Lines        5872     5873    +1     
=======================================
+ Hits         4671     4672    +1     
  Misses       1201     1201           

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[waahm7]

When the provider_user_data is NULL, we never trigger the callback is wrong. I think the right solution would be to just fatal assert on provider_user_data being not NULL in s_make_request function, but I haven't traced through all the code paths.