zmbvdec: check decompression buffer size.

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

libavcodec/zmbv.c

@@ -502,8 +502,11 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size, AVPac
     }
 
     if (c->comp == 0) { //Uncompressed data
+        if (c->decomp_size < len) {
+            av_log(avctx, AV_LOG_ERROR, "decomp buffer too small\n");
+            return AVERROR_INVALIDDATA;
+        }
         memcpy(c->decomp_buf, buf, len);
-        c->decomp_size = 1;
     } else { // ZLIB-compressed data
         c->zstream.total_in = c->zstream.total_out = 0;
         c->zstream.next_in = (uint8_t*)buf;