KCompressionDevice: don't call write after WriteError

Summary:
if an earlier call to write failed, then there is no point in
doing the final write call, it even asserts in the NoneFilter.

Test Plan:
This fixes saving to full disk using a KCompressionDevice set
up with KNoneFilter, in kwrite.

BUG: 397545

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D14920