chan_unistim: Unlock mutex in rare OOM condition.

ASTERISK-23792 #close
Reported by: Peter Whisker

Review: https://reviewboard.asterisk.org/r/3567/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@414677 f38db490-d61c-443f-a65b-d21fe96a405b