select: avoid a NULL deref in cwfds_add_sock

[bagder]

curl_multi_waitfds(m, NULL, ...);

=> Curl_waitfds_init(&cwfds, ufds, size);

=> Curl_waitfds_add_ps(&cwfds);

=> cwfds_add_sock(cwfds, ...);

Would then try to use the ->wfds array while set to NULL previously. This should not happen, why this is protected with an assert to trigger debug builds if it happens.

Caught by CodeSonar

[testclutch]

Analysis of PR #15881 at d5b7fbab:

Test 2405 failed, which has NOT been flaky recently, so there could be a real issue in this PR. Note that this test has failed in 18 different CI jobs (the link just goes to one of them).

Generated by Testclutch

[jay]

Would then try to use the ->wfds array while set to NULL previously. This should not happen, why this is protected with an assert to trigger debug builds if it happens.

My understanding is cwfds_add_sock already returns 1 if there is no array (count should be 0 in this case). The debugassert does not work (see CI failure) because cwfds_add_sock is expected to work with a possible NULL wfds but count is 0 so it's ok. I don't understand how codesonar thinks it could actually dereference?

/cc @chrisd-work

[bagder]

I don't understand how codesonar thinks it could actually dereference?

Presumably because it does not properly take all the additional conditions into account. I included a screenshot of its warning in the top for better explanation.