cleanups for commit from issue #5657... set a cause code for a rejected...

cleanups for commit from issue #5657... set a cause code for a rejected forward request, and actually set tmp->chan to NULL when we reject the forward request


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@31049 65c4cc65-6c06-0410-ace0-fbb531ad65f3