Addressing clang-UBSan Undefined Behavior Memcheck

[pegeler]

Purpose

This PR fixes undefined behavior uncovered by the CRAN clang-UBSan memcheck raised earlier this week. cran_comments.md goes into detail on how this bug can be reproduced.

Explanation

The bug was found and fixed in src/prime_factors.cpp. The prime_factors_ helper function takes an int input that is cast to double, passed to std::sqrt, then cast back to int. The return value of std::sqrt when encountering domain error (input < 0) is undefined but expected to be NaN, which cannot be represented when cast back to int. Thus we have undefined behavior.

The Fix

I have added an early exit for values that would cause a domain error.