h264_mp4toannexb: Add a comment about possible overread

Before reading a 16bit size field during parsing of extradata, no check
is performed to make sure that said length field is actually contained
in the extradata. Given that this overread is not dangerous (the extradata
is supposed to be padded), only a comment for it has been added; the error
itself will be detected as part of the normal check for overreads.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_mp4toannexb_bsf.c

@@ -98,6 +98,7 @@ static int h264_extradata_to_annexb(AVBSFContext *ctx, const int padding)
     while (unit_nb--) {
         int err;
 
+        /* possible overread ok due to padding */
         unit_size   = bytestream2_get_be16u(gb);
         total_size += unit_size + 4;
         av_assert1(total_size <= INT_MAX - padding);