chan_sip: Remove excess ref of realtime peer before sip_poke_peer.

The peer is referenced at the end of sip_poke_peer, it should not get
an extra ref before the call to sip_poke_peer. This fixes a memory
leak.

ASTERISK-22945 #close
Reported by: ibercom
Tested by: Yuriy Gorlichenko
Patches:
  asterisk11.patch uploaded by ibercom (License #6599)

Review: https://reviewboard.asterisk.org/r/4031/
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Merged revisions 424176 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 424177 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 424178 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424179 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@424180 f38db490-d61c-443f-a65b-d21fe96a405b