AST-2014-015: Fix race condition in chan_pjsip when sending responses after a CANCEL...

AST-2014-015: Fix race condition in chan_pjsip when sending responses after a CANCEL has been received.

Due to the serialized architecture of chan_pjsip there exists a race condition where a CANCEL may
be received and processed before responses (such as 180 Ringing, 183 Session Progress, and 200 OK)
are sent. Since the session is in an unexpected state PJSIP will assert when this is attempted.

This change makes it so that these responses are not sent on disconnected sessions.

ASTERISK-24471 #close
Reported by: yaron nahum

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@428301 65c4cc65-6c06-0410-ace0-fbb531ad65f3