Change ast_hangup() to return void and be NULL safe.

Change ast_hangup() to return void and be NULL safe.

Since ast_hangup() is effectively a channel destructor, it should be a
void function.

* Make the few silly callers checking the return value no longer do so.
Only the CDR and CEL unit tests checked the return value.

* Make all callers take advantage of the NULL safe change and remove the
NULL check before the call.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394623 65c4cc65-6c06-0410-ace0-fbb531ad65f3