Correctly set mateThreat in search()

We do not accept null search returned mate values,
but we always do a full search in those cases.

So the variable mateThreat that is set only if null move
search returns a mate value is always false.

Restore the functionality of mateThreat moving the
assignement where it can be triggered.

After 999 games at 1+0

Mod vs Orig +253 =517 -229 51.20%  +8 ELO

Bug reported by xiaozhi

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