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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@414678 f38db490-d61c-443f-a65b-d21fe96a405b