Situation:

• mongoc_client_reset is called on a mongoc_client_t
• an operation is performed on the client that requires popping a session from the session pool. This would be most operations against 3.6+ server.

Since _mongoc_topology_clear_session_pool does not NULL out topology->session_pool, the next attempt to pop a session results in a use-after-free when accessing topology->session_pool.