avformat/vpk: Check offset for validity

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit aa003019)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/vpk.c

@@ -66,6 +66,9 @@ static int vpk_read_header(AVFormatContext *s)
         return AVERROR_INVALIDDATA;
     vpk->block_count       = (st->duration + (samples_per_block - 1)) / samples_per_block;
     vpk->last_block_size   = (st->duration % samples_per_block) * 16 * st->codecpar->channels / 28;
+
+    if (offset < avio_tell(s->pb))
+        return AVERROR_INVALIDDATA;
     avio_skip(s->pb, offset - avio_tell(s->pb));
     avpriv_set_pts_info(st, 64, 1, st->codecpar->sample_rate);