valgrind confused by std::allocator calls to operator new(0)

[Benjamin Kosnik]

> I agree that calling new(0) is dumb and we should simply standardize on 
> the behavior of pool and mt: it's just a matter of adding a conditional 
> to new_allocator, seems safe for 3.4.4 too. Please file a PR, thanks.

Agreed. I'll fix this up.

-benjamin