avcodec/vp9_superframe_split_bsf: Discard invalid zero-sized frames

They are invalid in VP9. If any of the frames inside a superframe
had a size of zero, the code would either read into the next frame
or into the superframe index; so check for the length to stop this.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/vp9_superframe_split_bsf.c

@@ -70,7 +70,7 @@ static int vp9_superframe_split_filter(AVBSFContext *ctx, AVPacket *out)
                         frame_size |= bytestream2_get_byte(&bc) << (j * 8);
 
                     total_size += frame_size;
-                    if (frame_size < 0 || total_size > in->size - idx_size) {
+                    if (frame_size <= 0 || total_size > in->size - idx_size) {
                         av_log(ctx, AV_LOG_ERROR,
                                "Invalid frame size in a superframe: %d\n", frame_size);
                         ret = AVERROR(EINVAL);