chan_unistim: Unlock mutex in rare OOM condition.

#ASTERISK-23792 #close
Reported by: Peter Whisker

Review: https://reviewboard.asterisk.org/r/3567/
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Merged revisions 414677 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 414678 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@414679 65c4cc65-6c06-0410-ace0-fbb531ad65f3