Add overflow checks to money type input function

The money type input function did not have any overflow checks at all.
There were some regression tests that purported to check for overflow,
but they actually checked for the overflow behavior of the int8 type
before casting to money.  Remove those unnecessary checks and add some
that actually check the money input function.

Reviewed-by: Fabien COELHO <coelho@cri.ensmp.fr>