Remove an unnecessary check and clarify some weird code in Deflater.cpp.

More java.util.zip cleanup. We throw the IllegalStateException in Java.
Strictly, that's racy, and we might want to move all this checking into
toNativeZipStream, but we haven't made such arrangements in general,
and I'll do that as a follow-up if desired; this patch just ensures that
we're consistent.

Change-Id: I86e47461117b3747e6fad9fe344cd68029bb78b1