Fix compiler warnings

[jishi9]

There are two warning types being addressed (the only two warnings in fact, well done!):

• abs(size_t) is a no-op
• printf format for size_t should be %zu rather than %d

https://stackoverflow.com/a/2524675 http://www.cplusplus.com/reference/cstdio/printf/ https://linux.die.net/man/3/printf "z: A following integer conversion corresponds to a size_t or ssize_t argument."