PJSIP: Prevent crash on blind transfer

Blind transfers don't go too well with NULL channels which can occur if
the channel has already been transferred away.

(closes issue ASTERISK-23718)
Reported by: Jonathan Rose
........

Merged revisions 414948 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@414949 65c4cc65-6c06-0410-ace0-fbb531ad65f3