chan_iax2: Fix a segfault introduced by call ID logging

Didn't previously check that a non NULL IAX channel was stored in the array
at the requested position before attempting iax_pvt_callid_get

(closes issue ASTERISK-20145)
Reported by: Birger "WIMPy" Harzenetter


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370335 65c4cc65-6c06-0410-ace0-fbb531ad65f3