Make bms_prev_member work correctly with a 64 bit bitmapword

Make bms_prev_member work correctly with a 64 bit bitmapword

5c067521 erroneously had coded bms_prev_member assuming that a bitmapword
would always hold 32 bits and started it's search on what it thought was the
highest 8-bits of the word.  This was not the case if bitmapwords were 64
bits.

In passing add a test to exercise this function a little. Previously there was
no coverage at all.

David Rowly