Fix reading of book file

Bug is subtle because appears only under MSVC 32 bits in
optimized version, hence was missed before.

Bug is due to the fact that evaluation order of terms of a
sum is undefined by the standard, so in get_int() we have:

return 256 * get_int<n-1>() + bookFile.get();

And if get() is evaluated before get_int() we have a corrupted
key.

The patch rewrites the code in a more natural and predictable way.

No functional change.

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