Don't crash if chan2 does not exist (one-legged call situations, like echo)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@33110 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -12439,7 +12439,7 @@ static int handle_request_refer(struct sip_pvt *p, struct sip_request *req, int
 		return -1;
 	}
-	if (sipdebug && option_debug > 3)
+	if (current.chan2 && sipdebug && option_debug > 3)
 		ast_log(LOG_DEBUG, "Got SIP transfer, applying to bridged peer '%s'\n", current.chan2->name);
 	/* Stop music on hold on this channel */