app_stack: Add missing unlock in off-nominal path of STACK_PEEK function.

ASTERISK-23620 #close
Reported by: Bradley Watkins
Patches:
      ASTERISK-23620_unlock_oldlist.patch (license #5021) patch uploaded by Bradley Watkins
........

Merged revisions 412225 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@412226 65c4cc65-6c06-0410-ace0-fbb531ad65f3