Fix a race condition in res_pjsip_session with rapidly terminating the session.

Fix a race condition in res_pjsip_session with rapidly terminating the session.

The INVITE session state callback wrongly assumes that a session will always exist, but
when rapidly terminating the session this assumption goes out the window. As all handler
code for the INVITE session state callback requires the session it will now just exit
immediately if no session exists.

(closes issue ASTERISK-22668)
Reported by: John Bigelow
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Merged revisions 400872 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400873 65c4cc65-6c06-0410-ace0-fbb531ad65f3