Fix integer overflow caused by bad use of binary operator.

Fix integer overflow caused by bad use of binary operator.

Guess this may make the AI more accurate :)

Problem caught by gcc:

../../gnushogi/search.c: In function 'search':
../../gnushogi/search.c:887: warning: overflow in implicit constant conversion

A quick audit only revealed a place where another logical AND was
tested using a curious idiom, so let's make it consistent at the same
time.