chnan_sip: Set SIP_DEFER_BYE_ON_TRANSFER prior to calling bridge blind transfer

This patch moves setting SIP_DEFER_BY_ON_TRANSFER prior to calling
ast_bridge_transfer_blind. This prevents a BYE from being sent prior to the
NOTIFY request that informs the transferor if the transfer succeeded or failed.

This patch also clears said flag from the off nominal NOTIFY paths in the
local_attended_transfer code, as once we've sent the NOTIFY request it is safe
to send by the BYE request.

This was caught by the blind-transfer-accountcode test in the Asterisk Test
Suite.

(closes issue ASTERISK-23290)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3214/
........

Merged revisions 408069 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408070 65c4cc65-6c06-0410-ace0-fbb531ad65f3