Issue #7176 - Crash in expire_register (imported from 1.2)

This is a bad fix, since peer should never be 0. Why is it zero
on Royk's system? 


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@27928 65c4cc65-6c06-0410-ace0-fbb531ad65f3