When _mongoc_cursor_fetch_stream was modified in 9eba451 for CDRIVER-3893, a code path was introduced where the return value of mongoc_cluster_stream_for_server was no longer checked for a null value before access.

There are few conditions where mongoc_cluster_stream_for_server might return null, but I observed this through a segfault in PHPC's server-executeQuery-012.phpt test due a mismatched server hint and pinned client session.