Missing overflow checking

[gavinwahl]

It looks like this addition is missing an overflow check: https://github.com/ChristophBerg/postgresql-unit/blob/cf6728e3773e1ff4a1c67f850aa44338405abafe/unit.h#L116

Multiplying values with very high exponents could result in undefined behavior.

[df7cb]

Right. I'll see to get that fixed.