xl: Fix overreads

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 00aad121d8)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/xl.c

@@ -68,6 +68,12 @@ static int decode_frame(AVCodecContext *avctx,
     V = a->pic.data[2];
 
     stride = avctx->width - 4;
+
+    if (buf_size < avctx->width * avctx->height) {
+        av_log(avctx, AV_LOG_ERROR, "Packet is too small\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     for (i = 0; i < avctx->height; i++) {
         /* lines are stored in reversed order */
         buf += stride;