Add some checks for errors at initialization, though there's not
much that can be done with ENOMEM-type errors aside from
dropping clients.
We can also get rid of the scary FIXME for MemLevel=8. It was a
stupid error on my part in the original per-client deflate
stream implementation calling C::R::Z::{Inflate,Deflate} in
array context and getting the extra dualvar error code as a
string result, causing the {zin}/{zout} array refs to have
extra array elements.