Fixes https://github.com/tendermint/tmlibs/issues/55
MemDB previously mistakenly set the actual DB pointer to nil
although that side effect is not visible to the outside world
since p is an identifier within the scope of just that function
call. However, @melekes and I had a discussion in which we
came to the conclusion that Close for an in-memory DB should
instead be a noop and not cause any data loss. See the
discussion on https://github.com/tendermint/tmlibs/pull/56.
Emmanuel Odeke
Ethan Buchman