mov: reset dref_count on realloc to keep values consistent.

This fixes a potential crash.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
static int mov_read_avss(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    return mov_read_extradata(c, pb, atom, CODEC_ID_AVS);
}

static int mov_read_avss(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    return mov_read_extradata(c, pb, atom, CODEC_ID_AVS);
}
