avcodec/mpeg4videodec: Check available data before reading custom matrix

Fixes: out of array read
Fixes: 76c515fc3779d1b838667c61ea13ce92/asan_heap-oob_1fc0d07_8913_794a4629a264ebdb25b58d3a94ed1785.bit

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpeg4videodec.c

@@ -1881,6 +1881,10 @@ static int decode_vol_header(Mpeg4DecContext *ctx, GetBitContext *gb)
                 int last = 0;
                 for (i = 0; i < 64; i++) {
                     int j;
+                    if (get_bits_left(gb) < 8) {
+                        av_log(s->avctx, AV_LOG_ERROR, "insufficient data for custom matrix\n");
+                        return AVERROR_INVALIDDATA;
+                    }
                     v = get_bits(gb, 8);
                     if (v == 0)
                         break;
@@ -1904,6 +1908,10 @@ static int decode_vol_header(Mpeg4DecContext *ctx, GetBitContext *gb)
                 int last = 0;
                 for (i = 0; i < 64; i++) {
                     int j;
+                    if (get_bits_left(gb) < 8) {
+                        av_log(s->avctx, AV_LOG_ERROR, "insufficient data for custom matrix\n");
+                        return AVERROR_INVALIDDATA;
+                    }
                     v = get_bits(gb, 8);
                     if (v == 0)
                         break;