[PATCH] bms_prev_member() can read beyond the end of the array of allocated words

Hello,

I've been working on Bitmapset and while creating a test suite for it I
found that there is a missing bounds check in bms_prev_member(). The
function takes the prevbit argument and converts it to an index into the
words array using WORDNUM() without checking to ensure that prevbit is
within the bounds of the possible values (e.g. nwords *
BITS_PER_BITMAPWORD) in the set. This means that $subject resulting in
a confusing return value when the expected value should be the highest
bit set.

The patch attached adds a bounds check preventing this.

-greg