B.xs: remove redundant assignment

Follow-up to 500f40f555ee.
NDBM: silence 'unused var' warnings x 2

(I've done this blind because I don't have libndbm on my system)
ODBM: fix a compiler warning.

This has been applied blind since my system doesn't have libodbm,
but it matches exactly the cast added to NDBM's typemap a few years ago to
fix a similar warning.
fix a couple of win32 op.c warnings

These warnings appear on win32 smokes.

op.c(8600) : warning C4244: 'function' : conversion from 'cv_flags_t' to 'char'
, possible loss of data
op.c(11905) : warning C4146: unary minus operator applied to unsigned type, res
ult still unsigned

Since I'm not running windows, I can't confirm that this commit shuts up
these warnings, but I think think I've understood them correctly.