Fix a couple of misuages of bms_num_members()

I noticed today there are a few places where we use bms_num_memebers()
where we really should be using bms_membership().  These are not bugs,
they're mostly just bad examples to leave laying around, at best, and
a small performance penalty, at worst.

Unless there are any objections, I plan to push this to master only in
about 10 hours time.

David