Don't crash on a guest directmedia call

Don't crash on a guest directmedia call

A sip_pvt may not have relatedpeer set if a call doesn't match up
with a peer. If there is no relatedpeer, there is no direct media
ACL to apply, so just return that it is allowed.

(closes issue ASTERISK-20040)
Reported by: Terry Wilson
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Merged revisions 369214 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369215 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369217 65c4cc65-6c06-0410-ace0-fbb531ad65f3