avcodec/hevc_mp4toannexb_bsf: Check nalu_size

Fixes: Timeout (29sec -> 5ms)
Fixes: 20237/clusterfuzz-testcase-minimized-ffmpeg_BSF_HEVC_MP4TOANNEXB_fuzzer-5165615044362240

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

libavcodec/hevc_mp4toannexb_bsf.c

@@ -144,6 +144,11 @@ static int hevc_mp4toannexb_filter(AVBSFContext *ctx, AVPacket *out)
         for (i = 0; i < s->length_size; i++)
             nalu_size = (nalu_size << 8) | bytestream2_get_byte(&gb);
 
+        if (nalu_size < 2) {
+            ret = AVERROR_INVALIDDATA;
+            goto fail;
+        }
+
         nalu_type = (bytestream2_peek_byte(&gb) >> 1) & 0x3f;
 
         /* prepend extradata to IRAP frames */