Fix a crash that occurred when a BYE was received on a replaced dialog.

Reference counting for the channel and its tech_pvt got messed up at
some point between 1.8 and 11. The result was that if a BYE for a dialog
that had been replaced (via an INVITE with Replaces) was received, Asterisk
would crash due to trying to access data on a channel that was no longer there.

The fix I introduced is to remove code that both unrefs the sip_pvt and sets
the channel's tech_pvt to NULL when an INVITE with Replaces is handled. This
way when a BYE is received, the tech_pvt will be non-NULL and so the BYE can
be processed and not cause a crash.

(closes issue ASTERISK-20929)
reported by Kristopher Lalletti
patches:
	ASTERISK-20929.patch uploaded by Mark Michelson (License #5049)



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@381566 f38db490-d61c-443f-a65b-d21fe96a405b