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: https://origsvn.digium.com/svn/asterisk/trunk@394278 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -18141,7 +18141,7 @@ static int get_refer_info(struct sip_pvt *transferer, struct sip_request *outgoi
 	/* Either an existing extension or the parking extension */
 	if (refer->attendedtransfer || ast_exists_extension(NULL, transfer_context, refer_to, 1, NULL)) {
 		if (sip_debug_test_pvt(transferer)) {
-			ast_verbose("SIP transfer to extension %s@%s by %s\n", refer_to, transfer_context, referred_by_uri);
+			ast_verbose("SIP transfer to extension %s@%s by %s\n", refer_to, transfer_context, S_OR(referred_by_uri, "Unknown"));
 		}
 		/* We are ready to transfer to the extension */
 		return 0;