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