vmd: decode videos with no LZ buffer size provided - they might not need it

The buffer is used for an additional pass of frame compression, so videos
can be coded without ever using it (and some are coded so indeed, e.g. in
Woodruff and the Schnibble of Azimuth game).

libavcodec/vmdav.c

@@ -278,6 +278,11 @@ static int vmd_decode(VmdVideoContext *s, AVFrame *frame)
         return AVERROR_INVALIDDATA;
     meth = bytestream2_get_byteu(&gb);
     if (meth & 0x80) {
+        if (!s->unpack_buffer_size) {
+            av_log(s->avctx, AV_LOG_ERROR,
+                   "Trying to unpack LZ-compressed frame with no LZ buffer\n");
+            return AVERROR_INVALIDDATA;
+        }
         lz_unpack(gb.buffer, bytestream2_get_bytes_left(&gb),
                   s->unpack_buffer, s->unpack_buffer_size);
         meth &= 0x7F;
@@ -389,9 +394,11 @@ static av_cold int vmdvideo_decode_init(AVCodecContext *avctx)
     vmd_header = (unsigned char *)avctx->extradata;
 
     s->unpack_buffer_size = AV_RL32(&vmd_header[800]);
-    s->unpack_buffer = av_malloc(s->unpack_buffer_size);
-    if (!s->unpack_buffer)
-        return -1;
+    if (s->unpack_buffer_size) {
+        s->unpack_buffer = av_malloc(s->unpack_buffer_size);
+        if (!s->unpack_buffer)
+            return AVERROR(ENOMEM);
+    }
 
     /* load up the initial palette */
     raw_palette = &vmd_header[28];