Fix error paths in action_hangup() for AMI Hangup action.

* Check allocation function return values for failure.  Crashing is bad.

* Tweak ast_regex_string_to_regex_pattern() parameters for proper ast_str 
usage.  


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368714 65c4cc65-6c06-0410-ace0-fbb531ad65f3