avformat/oggdec: Don't skip over data whose checksum is used

The behavior of the ffio_*_checksum feature is not well defined
when using avio_skip(). The code in oggdec.c relied on the skipped
data (four bytes) to be checksummed, which is mostly true
because short_seek_threshold is 32768 by default, so that
avio_seek() will normally read data instead of calling the
underlying seek function. Yet this has two problems:

a) It relies on implementation details of avio_seek().
b) There is an exception, namely if the AVIO_FLAG_DIRECT is set.
In this case the underlying seek function (if set) is always
called and the data is skipped, leading to CRC errors.

So don't skip the data.

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

libavformat/oggdec.c

@@ -370,7 +370,7 @@ static int ogg_read_page(AVFormatContext *s, int *sid, int probing)
     flags   = avio_r8(bc);
     gp      = avio_rl64(bc);
     serial  = avio_rl32(bc);
-    avio_skip(bc, 4); /* seq */
+    avio_rl32(bc); /* seq */
 
     crc_tmp = ffio_get_checksum(bc);
     crc     = avio_rb32(bc);