Undefined behavior: Upper bound of zero

[48656c6c6f]

When a function such as pcg_setseq_64_xsh_rr_32_boundedrand_r is called with a bound of 0 and bound is the denominator of a modulus operation, the result is undefined behavior -- possibly a division by zero error.

Such functions should check for and return zero in such cases to avoid undefined behavior.

[lemire]

As an aside, these functions can be optimized to require just one division instead of two. @imneme knows how!