lavf: Fix try_decode_frame() so it doesnt loop infinitely.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2273,6 +2273,8 @@ static int try_decode_frame(AVStream *st, AVPacket *avpkt, AVDictionary **option
             pkt.size -= ret;
         }
     }
+    if(!pkt.data && !got_picture)
+        return -1;
     return ret;
 }