avformat/mov: take into account the first eight bytes in the keys atom

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/mov.c

@@ -5024,6 +5024,7 @@ static int mov_read_keys(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     avio_skip(pb, 4);
     count = avio_rb32(pb);
+    atom.size -= 8;
     if (count > UINT_MAX / sizeof(*c->meta_keys) - 1) {
         av_log(c->fc, AV_LOG_ERROR,
                "The 'keys' atom with the invalid key count: %"PRIu32"\n", count);