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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@412225 f38db490-d61c-443f-a65b-d21fe96a405b