Fix DIVIDE BY ZERO exception in init_search()

It happens that when d == 0 we calculate:

log(double(0 * 0) / 2)

Unfortunately, log(0) is "illegal" and can generate either a
floating point exception or return a nonsense "huge" value
depending on the platform.

This fixs in the proper way the GCC/ICC rounding difference,
bug was from our side, not in the intel compiler.

Also fixed some few other warnings.

Bug spotted by Richard Lloyd.

No functional change.

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

src/pawns.cpp

@@ -148,7 +148,7 @@ PawnInfo* PawnInfoTable::get_pawn_info(const Position& pos) const {
   assert(pos.is_ok());
 
   Key key = pos.get_pawn_key();
-  int index = int(key & (size - 1));
+  unsigned index = unsigned(key & (size - 1));
   PawnInfo* pi = entries + index;
 
   // If pi->key matches the position's pawn hash key, it means that we