chan_sip.c: Moved some sip_pvt unrefs after their last use.

* Moved sip_pvt unref in ast_hangup() and handle_request_do() to the end
of the function.  The unref needs to happen after the last use of the
pointer.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@412348 f38db490-d61c-443f-a65b-d21fe96a405b