Fix a crash in res_pjsip_t38 caused by the wrong assumption that a session...

Fix a crash in res_pjsip_t38 caused by the wrong assumption that a session will always have a channel.

When starting up or shutting down this assumption is false.
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Merged revisions 400284 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400285 65c4cc65-6c06-0410-ace0-fbb531ad65f3