Fix overflow check insufficiently improved in r19840.

It assumes that sizeof(vmd_frame) < 64k, otherwise an additional
check to ensure sound_buffers <= UINT_MAX / sizeof(vmd_frame) would be necessary.

Originally committed as revision 19882 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/sierravmd.c

@@ -161,7 +161,7 @@ static int vmd_read_header(AVFormatContext *s,
     vmd->frame_table = NULL;
     sound_buffers = AV_RL16(&vmd->vmd_header[808]);
     raw_frame_table_size = vmd->frame_count * 6;
-    if(vmd->frame_count * vmd->frames_per_block  >= (UINT_MAX - sound_buffers) / sizeof(vmd_frame)){
+    if(vmd->frame_count * vmd->frames_per_block >= UINT_MAX / sizeof(vmd_frame) - sound_buffers){
         av_log(s, AV_LOG_ERROR, "vmd->frame_count * vmd->frames_per_block too large\n");
         return -1;
     }