mdb_cursor_prev/mdb_cursor_last() with data==NULL position the
xcursor at 1st data item, while data!=NULL puts it at last item.
That's unintuitive and should either be documented or normalized -
but changing it can break existing programs. In the cases when
it didn't just fail earlier, anyway: mdb_cursor_prev(), and maybe
mdb_cursor_last() on a sub-DB. Haven't tested the latter.
Whatever mdb_cursor_last() ends up doing, I think mdb_cursor_prev()
should do the same.
There is positioning code inside if(data) elsewhere too, but
defaults to doing cursor_first() or something similar. Don't know
if that can have any visible effect on the next operation.
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