chan_sip: Unref outbound proxy structure on dialog/pvt destruction.

Make sure outbound proxy refs are always unreffed on dialog destruction.

Review: https://reviewboard.asterisk.org/r/4016/
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Merged revisions 423800 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 423801 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 423802 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423803 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423804 65c4cc65-6c06-0410-ace0-fbb531ad65f3