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.
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Merged revisions 412348 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 412383 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@412384 65c4cc65-6c06-0410-ace0-fbb531ad65f3