bdb: don't retrieve the db values when warning about nil key

bdb: don't retrieve the db values when warning about nil key

When either of the keys passed to the BDB compare function is nil, we
emit a warning. In this warning, however, we used to peek at the
registry value, which required the use of the comparison function
itself, falling in a nested loop. Solve by omitting the
self[<key>].inspect from the warning.