avformat/ipmovie: check OPCODE_INIT_VIDEO_BUFFERS size more completely

Fixes use of uninitialized data

Fixes: signal_sigsegv_1571228_5930_ipmovie_interplayvideo_interplay_dpcm__bislogo.mve

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/ipmovie.c

@@ -376,7 +376,7 @@ static int process_ipmovie_chunk(IPMVEContext *s, AVIOContext *pb,
 
         case OPCODE_INIT_VIDEO_BUFFERS:
             av_dlog(NULL, "initialize video buffers\n");
-            if ((opcode_version > 2) || (opcode_size > 8)) {
+            if ((opcode_version > 2) || (opcode_size > 8) || opcode_size < 4) {
                 av_dlog(NULL, "bad init_video_buffers opcode\n");
                 chunk_type = CHUNK_BAD;
                 break;