Pretty up a debug message if the referred-by-uri isn't available

Instead of formatting a NULL pointer into a "%s" format string (which is
usually not a good thing to do), we instead print "Unknown".


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@394278 f38db490-d61c-443f-a65b-d21fe96a405b