bpo-44285: getpath.c: Assert that env_file is NULL during an error check (GH-26486)

This was flagged by a static analyzer, but the logic of why this is NULL on error is hard to follow for humans as well.
(cherry picked from commit bdb56902a3bfe12b10f85a941d5dd0eae739f1a8)

Co-authored-by: stratakis <cstratak@redhat.com>