Re: Strange Bitmapset manipulation in DiscreteKnapsack()

While working on [1], I noticed some strange code in
DiscreteKnapsack() which seems to be aiming to copy the Bitmapset.

It's not that obvious at a casual glance, but:

sets[j] = bms_del_members(sets[j], sets[j]);

this is aiming to zero all the words in the set by passing the same
set in both parameters.

Now that 00b41463c changed Bitmapset to have NULL be the only valid
representation of an empty set, this code no longer makes sense.  We
may as well just bms_free() the original set and bms_copy() in the new
set as the bms_del_members() call will always pfree the set anyway.

I've done that in the attached.