can actually manage to ask createStream for missing streams

writerperfect::(anonymous namespace)::OLEStorageImpl::createStream(rtl::OUString const&) (WPXSvInputStream.cxx:260)
writerperfect::WPXSvInputStreamImpl::getSubStreamByName(char const*) (WPXSvInputStream.cxx:201)
MWAWInputStream::unzipStream()
MWAWInputStream::MWAWInputStream

valgrind + bff

Change-Id: Iaecb8b7f05ed4369ddd37b34166dc3c3388ffb4d