Fix exporting to non-native formats on Windows, e.g. HTML

The fix follows approach of bf47fed5ead06f2128abf042897f86bab39cf333

Also fixes a possible crash on Windows in KoFilterChain::outputFileHelper(...):
m_outputTempFile was accessed even when it might be set to 0 a few lines before

REVIEW: 107388

Thanks boemann and inge for review and stuart for package building