Reset bestMove before entering main moves loop

Reset bestMove before entering main moves loop

After razoring, IID, null verification and singular
extension searches we have could have a dirty ss->bestMove,
restore to MOVE_NONE before to enter moves loop.

This should avoid to store in TT a stale move when
we fail low.

Tested together with previous patch that is the
one that gives ELO.

After 1152 games at 1+0 on my QUAD
Mod vs Orig +233 =716 -203 (+9 ELO)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>