pjsip_transport_events.c: Fix crash using stale transport pointer.

pjsip_transport_events.c: Fix crash using stale transport pointer.

Apparently it is possible for the transport to be destroyed without
triggering the transport callback logic.  As a result the transport gets
destroyed and we have a stale pointer in the active_transports container.

* Invoke the transport monitor callback checks when the transport is
destroyed in addition to when it is disconnected and shutdown.

ASTERISK-27688

Change-Id: Ia9b5469fea8f2b3f2d8476fae6b748a4d23e7261