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/
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Merged revisions 403587 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403588 65c4cc65-6c06-0410-ace0-fbb531ad65f3