Add some safety measures when using gosub, especially when using the options

for app_dial and app_queue to run a gosub when the call is answered.

* Check for the existence of the gosub target in gosub_exec. If it is nonexistent,
  then this will cause errors when we attempt to actually run the gosub, including
  a definite memory leak and potential crashes. Return an error in this situation
* Check the return value of pbx_exec in app_dial and app_queue before attempting
  to actually run the gosub routine. If there was an error, we should not attempt
  to run the gosub.
* Change a '|' to a ',' in app_queue.
* Add some extra curly braces where they had been missing previously.

(closes issue #13548)
Reported by: fiddur



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@160626 65c4cc65-6c06-0410-ace0-fbb531ad65f3