Avoid unconditional NULLing of mwipvt on relatedpeer on SIP dialog destruction

Avoid unconditional NULLing of mwipvt on relatedpeer on SIP dialog destruction

The other instance of this bug was fixed by jcolp/file in r121496. If
we are destroying a dialog only set the MWI dialog pointer on the
related peer to NULL if it is the dialog currently being destroyed.

(closes issue ASTERISK-20119)
Patch-by: Misha Vodsedalek
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Merged revisions 371270 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371271 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371272 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371277 65c4cc65-6c06-0410-ace0-fbb531ad65f3