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

apps/app_stack.c

@@ -852,6 +852,7 @@ static int stackpeek_read(struct ast_channel *chan, const char *cmd, char *data,
 		if (!ast_true(args.suppress)) {
 			ast_log(LOG_ERROR, "Stack peek of '%s' is more stack frames than I have\n", args.n);
 		}
+		AST_LIST_UNLOCK(oldlist);
 		ast_channel_unlock(chan);
 		return -1;
 	}