do not ast_hangup() on a NULL channel.

In the original code this would happen in the case of
	o->forwards >=  AST_MAX_FORWARDS

Likely an 1.2/1.4 isse as well - please someone have a look,
while I am hunting a few more similar panics now.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@47880 65c4cc65-6c06-0410-ace0-fbb531ad65f3