check udp_read_packet return value not to access uninitialized memory

patch by Eric Paul: [eric paul gmail com]
original thread: [FFmpeg-devel] [PATCH] : libavformat segfault when a
RTSP reading is interrupted
date: 07/05/2007 03:13 PM

Originally committed as revision 9492 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtsp.c

@@ -1194,7 +1194,7 @@ static int rtsp_read_packet(AVFormatContext *s,
     case RTSP_PROTOCOL_RTP_UDP:
     case RTSP_PROTOCOL_RTP_UDP_MULTICAST:
         len = udp_read_packet(s, &rtsp_st, buf, sizeof(buf));
-        if (rtsp_st->rtp_ctx)
+        if (len >=0 && rtsp_st->rtp_ctx)
             rtp_check_and_send_back_rr(rtsp_st->rtp_ctx, len);
         break;
     }