Don't try to read from an rtcp if there isn't one

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5934 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rtp.c

@@ -325,7 +325,7 @@ struct ast_frame *ast_rtcp_read(struct ast_rtp *rtp)
 	unsigned int rtcpdata[1024];
 	char iabuf[INET_ADDRSTRLEN];
 	
-	if (!rtp)
+	if (!rtp || !rtp->rtcp)
 		return &null_frame;
 
 	len = sizeof(sin);