Fix memory leaks and deadlocks in chan_unistim.

(closes issue #11158)
Reported by: eliel
Patches:
      chan_unistim.c.patch uploaded by eliel (license 64)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@88510 65c4cc65-6c06-0410-ace0-fbb531ad65f3