chan_pjsip: Fix a sticking channel lock caused by channel masquerades

(closes issue ASTERISK-22936)
Reported by: Jonathan Rose
Review: https://reviewboard.asterisk.org/r/3042/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@403587 f38db490-d61c-443f-a65b-d21fe96a405b