Fix a race when extracting PV from TT

Fix a race when extracting PV from TT

Because TT table is shared tte->move() could change
under our feet, in particular we could validate
tte->move() then the move is changed by another
thread and we call pos.do_move() with a move different
from the original validated one !

This leads to a very rare but reproducible crash once
every about 20K games.

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