Store score in TT when null search fails high

Use full depth, not reduced one. This allows
to avoid to do a null search when in the same
position and at the same or bigger depth the
null search failed high.

A very small increase, if any.

After 963 games at 1+0
Mod vs Orig: +158 =657 -147  +4 ELO

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