Fix reference leaks involving SIP Replaces transfers

The reference held for SIP blind transfers using the Replaces header in an
INVITE was never freed on success and also failed to be freed in some error
conditions.  This caused a file descriptor leak since the RTP structures in use
at the time of the transfer were never freed.  This reference leak and another
relating to subscriptions in the same code path have now been corrected.

(closes issue ASTERISK-19579)
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Merged revisions 363986 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 363987 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@363988 65c4cc65-6c06-0410-ace0-fbb531ad65f3