Add Asserts to validate prevbit values in bms_prev_member

Add Asserts to validate prevbit values in bms_prev_member

bms_prev_member() could attempt to access memory outside of the words[]
array in cases where the prevbit was a number < -1 or > a->nwords *
BITS_PER_BITMAPWORD + 1.

Here we add the Asserts to help draw attention to bogus callers so we're
more likely to catch them during development.

In passing, fix wording of bms_prev_member's header comment which talks
about how we expect the callers to ensure only valid prevbit values are
used.

Author: Greg Burd <greg@burd.me>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2000A717-1FFE-4031-827B-9330FB2E9065%40getmailspring.com