fix: handle taxa count divisible by 32

[magnified103]

The following snippet crashes under -fsanitize=undefined when the number of taxa is a multiple of 32.

iqtree3/pda/split.cpp

Lines 179 to 185 in 7cded0c

	for (iterator it = begin(), sit = sp.begin(); it != end(); it++, sit++)
	{
	int num_bits = (it+1 == end()) ? ntaxa % UINT_BITS : UINT_BITS;
	UINT it2 = (1 << (num_bits-1)) - 1 + (1 << (num_bits-1)) - (*it);
	UINT sit2 = (1 << (num_bits-1)) - 1 + (1 << (num_bits-1)) - (*sit);
	res |= (*it) & (*sit);

The fix simply sets num_bits to 32 in that case. It's similar to the fix in iqtree/iqtree2#412. I also manually checked all relevant parts in this file using the same logic.

I think we also need to backport this fix to IQ-TREE 2.

[bqminh]

thanks for discovering this!