Avoid overflows when summing 32-bit integers

[wegank]

This change mirrors the following (though the signedness doesn't matter, I guess)...

msolve/src/neogb/f4sat.c

Lines 222 to 234 in 079d814

	static inline unsigned long sum(
	const len_t * const ind,
	const len_t len)
	{
	len_t i;
	unsigned long sum = 0;
	for (i = 0; i < len; i++){
	sum += ind[i];
	}
	return sum;
	}

Resolves #125.

[jerebertho]

Many thanks!

[jerebertho]

Looks good to me, thanks!