trivial: fix bloom filter init to isEmpty = true #9060

Fixes newly initialized bloom filters being constructed with isEmpty(false), which still works but loses the possible speedup when checking for key membership in an empty filter.

I don't see anything wrong with doing this, though the IsEmpty/IsFull optimizations were largely a pretextual optimization intended to make unexploitable a remote crash vulnerability (integer division by zero) that existed in the original bloom filtering code without disclosing it. I'm doubtful that they are all that useful. :)

(if someone does want to remove them entirely, make sure to actually remove the vulnerability too! :) )

We need to be better about documenting such failures after sufficient time has passed...

On November 1, 2016 9:39:50 PM EDT, Gregory Maxwell notifications@github.com wrote:

I don't see anything wrong with doing this, though the IsEmpty/IsFull optimizations were largely a pretextual optimization intended to make unexploitable a remote crash vulnerability (integer division by zero) that existed in the original bloom filtering code without disclosing it.

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utACK https://github.com/bitcoin/bitcoin/pull/9060/commits/cccf73db0483cc3945bf8389ce197df35e931e16, as long as we still have these optimizations they should at least be correct.