Smarten mtv's probe a bit. Check for non zero

on bpp and width|height header fields. This
change is among the ones suggested by Vitor
Sessak in his '[FFmpeg-devel] [PATCH] Improve
MTV probe' thread.

Originally committed as revision 20283 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mtv.c

@@ -25,6 +25,7 @@
  */
 
 #include "libavutil/bswap.h"
+#include "libavutil/intreadwrite.h"
 #include "avformat.h"
 
 #define MTV_ASUBCHUNK_DATA_SIZE 500
@@ -56,6 +57,10 @@ static int mtv_probe(AVProbeData *p)
     if(*(p->buf) != 'A' || *(p->buf+1) != 'M' || *(p->buf+2) != 'V')
         return 0;
 
+    /* Check for nonzero in bpp and (width|height) header fields */
+    if(!(p->buf[51] && AV_RL16(&p->buf[52]) | AV_RL16(&p->buf[54])))
+        return 0;
+
     return AVPROBE_SCORE_MAX;
 }