Fix unlikely overflow bug in bms_next_member()

Fix unlikely overflow bug in bms_next_member()

... and bms_prev_member().

Both of these functions won't work correctly when given a prevbit of
INT_MAX and would crash when operating on a Bitmapset that happened to
have a member with that value.

Here we fix that by using an unsigned int to calculate which member to
look for next.

I've also adjusted bms_prev_member() to check for < 0 rather than == -1
for starting the loop.  This was done as it's safer and comes at zero
extra cost.

With our current use cases, it's likely impossible to have a Bitmapset
with an INT_MAX member, so no backpatch here.  I only noticed this issue
when working on a bms function to bitshift a Bitmapset.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAApHDvr1B2gbf6JF69QmueM2QNRvbQeeKLxDnF=w9f9--022uA@mail.gmail.com