Correct a bug introduced by Stéphane in the previous patch.

When we are using the "Bitboard + Square" overloaded operators,
the compiler uses the interpediate SquareBB[s] to transform the
square into a Bitboard, and then calculate the result.

For instance, the following code:

```
   b = pos.pieces(Us, PAWN) & s
```

generates in fact the code:

```
   b = pos.pieces(Us, PAWN) & SquareBB[s]`
```

The bug introduced by Stéphane in the previous patch was the
use of `b = pos.pieces(Us, PAWN) & (s + Up)` which can result
in out-of-bounds errors for the SquareBB[] array if s in the
last rank of the board.

We coorect the bug, and also add some asserts in bitboard.h to
make the code more robust for this particular bug in the future.

Bug report by Joost VandeVondele. Thanks!

Bench: 5512000