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: http://svn.asterisk.org/svn/asterisk/trunk@394623 f38db490-d61c-443f-a65b-d21fe96a405b