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_chap(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    c->chapter_track = avio_rb32(pb);
    return 0;
}

static int mov_read_chap(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    c->chapter_track = avio_rb32(pb);
    return 0;
}
