Make Deflater.cpp more like Inflater.cpp.

I didn't have a bug as motivation for this (Z_NEED_DICT isn't relevant
for deflation), but I want the deflation code to look at least as sane
as the inflation code. This still passes our tests, boots, and seems
to work okay. I think the "!= Z_FINISH" test was just a premature
optimization rather than anything meaningful.

Change-Id: I8db567605712a079aaf9e18cf8836c8d5673edb1