leveldown: error if I close while a create read stream is running/started

this comes in a few flavors, if you create close before the stream is ‘ready’, then it will not have an iterator. (this is probably fixed by my previous commit, that is coming into 0.9)

next, if it happens while the stream is still reading, readStream tries to call next() after end().

I’m gonna get something to eat, and then I’ll add a test for this after.