avcodec/jpeg2000dec: Check that atom header is within bytsetream

Fixes: Infinite loop
Fixes: 36666/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_JPEG2000_fuzzer-5912760671141888

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/jpeg2000dec.c

@@ -2360,6 +2360,8 @@ static int jp2_find_codestream(Jpeg2000DecoderContext *s)
                    atom_size >= 16) {
             uint32_t atom2_size, atom2, atom2_end;
             do {
+                if (bytestream2_get_bytes_left(&s->g) < 8)
+                    break;
                 atom2_size = bytestream2_get_be32u(&s->g);
                 atom2      = bytestream2_get_be32u(&s->g);
                 atom2_end  = bytestream2_tell(&s->g) + atom2_size - 8;