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 av_always_inline int dctcoef_get(int16_t *mb, int high_bit_depth,
                                        int index)
{
    if (high_bit_depth) {
        return AV_RN32A(((int32_t *)mb) + index);
    } else
        return AV_RN16A(mb + index);
}

static av_always_inline int dctcoef_get(int16_t *mb, int high_bit_depth,
                                        int index)
{
    if (high_bit_depth) {
        return AV_RN32A(((int32_t *)mb) + index);
    } else
        return AV_RN16A(mb + index);
}
