if ( mv_diff < 0 ) return v;
static int decode_mv_component(GetBitContext *gb, int v){    int mv_diff = get_vlc2(gb, h261_mv_vlc.table, H261_MV_VLC_BITS, 2);    mv_diff = mvmap[mv_diff];    if(mv_diff && !get_bits1(gb))        mv_diff= -mv_diff;        v += mv_diff;    if     (v <=-16) v+= 32;    else if(v >= 16) v-= 32;    return v;}