avcodec/c93: Check for block overlap.

Fixes overlapping memcpy()

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

libavcodec/c93.c

@@ -168,7 +168,14 @@ static int decode_frame(AVCodecContext *avctx, void *data,
             case C93_4X4_FROM_PREV:
                 for (j = 0; j < 8; j += 4) {
                     for (i = 0; i < 8; i += 4) {
-                        offset = bytestream2_get_le16(&gb);
+                        int offset = bytestream2_get_le16(&gb);
+                        int from_x = offset % WIDTH;
+                        int from_y = offset / WIDTH;
+                        if (block_type == C93_4X4_FROM_CURR && from_y == y+j &&
+                            (FFABS(from_x - x-i) < 4 || FFABS(from_x - x-i) > WIDTH-4)) {
+                            avpriv_request_sample(avctx, "block overlap %d %d %d %d\n", from_x, x+i, from_y, y+j);
+                            return AVERROR_INVALIDDATA;
+                        }
                         if ((ret = copy_block(avctx, &out[j*stride+i],
                                               copy_from, offset, 4, stride)) < 0)
                             return ret;