Remove unnecessary zero checks for unsigned types

[NewProggie]

std::size_t is an unsigned integer type which can't hold a value below zero. Therefore checking for < 0 is unnecessary. Removing so called tautological expressions which are always true as well.

[ftomassetti]

Right, and we should also move away from size_t (unsigned int seems more reasonable).

Thank you a lot!

[psi29a]

Just set: Skip branches without appveyor.yml

[ftomassetti]

thanks!