avcodec/agm: Include block size in the MV check for flags == 3

Fixes: out of array read
Fixes: 19331/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5644115983466496

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/agm.c

@@ -423,8 +423,8 @@ static int decode_inter_plane(AGMContext *s, GetBitContext *gb, int size,
                 int map = s->map[x];
 
                 if (orig_mv_x >= -32) {
-                    if (y * 8 + mv_y < 0 || y * 8 + mv_y >= h ||
-                        x * 8 + mv_x < 0 || x * 8 + mv_x >= w)
+                    if (y * 8 + mv_y < 0 || y * 8 + mv_y + 8 >= h ||
+                        x * 8 + mv_x < 0 || x * 8 + mv_x + 8 >= w)
                         return AVERROR_INVALIDDATA;
 
                     copy_block8(frame->data[plane] + (s->blocks_h - 1 - y) * 8 * frame->linesize[plane] + x * 8,