Allow SIP pvts involved in Replaces transfers to fall out of reference sooner

Allow SIP pvts involved in Replaces transfers to fall out of reference sooner

Unref the SIP pvt stored in the refer structure as soon as it is no longer
needed so that the pvt and associated file descriptors can be freed sooner.
This change makes a reference decrement unnecessary in code that handles SIP
BYE/Also transfers which should not touch the reference anyway.

(Closes issue ASTERISK-19579)
Reported by: Maciej Krajewski
Tested by: Maciej Karjewski

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@364258 65c4cc65-6c06-0410-ace0-fbb531ad65f3