avcodec/h264: do not trust last_pic_droppable when marking pictures as done

This simplifies the code and fixes a deadlock

Fixes Ticket2927
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
static inline int pic_is_unused(H264Context *h, Picture *pic)
{
    if (pic->f.data[0] == NULL)
        return 1;
    if (pic->needs_realloc && !(pic->reference & DELAYED_PIC_REF))
        return 1;
    return 0;
}

static inline int pic_is_unused(H264Context *h, Picture *pic)
{
    if (pic->f.data[0] == NULL)
        return 1;
    if (pic->needs_realloc && !(pic->reference & DELAYED_PIC_REF))
        return 1;
    return 0;
}
