[refactor] adding assertion of underflow handling

[At-sushi]

Features

• adding an assertion to the bound of an argument

The behavior of underflowing is well-defined (C99 7.15.1.1 §2) but I thought it is better to assert.

[yosupo06]

Currently we don't plan to use assertion except to check constraints for user inputs.

If we add assertion to internal... probably we will introduce it together with #ifdef ATCODEDR_INTERNAL_DEBUG or something, but not determined yet.