PJSIP FAX: Fix T.38 automatic reject timer NULL channel pointer dereferences.

When a caller calls a FAX number and then hangs up right after the call is
answered then the T.38 re-INVITE automatic reject timer may still be
running after the channel goes away.

* Added session NULL channel checks on the code paths that get executed by
t38_automatic_reject() to prevent a crash when the T.38 re-INVITE
automatic reject timer expires.

ASTERISK-25168
Reported by: Carl Fortin

Change-Id: I07b6cd23815aedce5044f8f32543779e2f7a2403