Archive FIlter: Crash when using okf_openxml

[Chase Tingley]

Running the Archive filter with a configuration that includes okf_openxml will produce a crash when the filter encounters an Office doc in the archive:

java.lang.NullPointerException
at net.sf.okapi.filters.archive.ArchiveFilter.nextInSubDocument(ArchiveFilter.java:385)
at net.sf.okapi.filters.archive.ArchiveFilter.next(ArchiveFilter.java:217)
at net.sf.okapi.steps.common.RawDocumentToFilterEventsStep.handleEvent(RawDocumentToFilterEventsStep.java:166)
at net.sf.okapi.common.pipeline.Pipeline.execute(Pipeline.java:117)
at net.sf.okapi.common.pipeline.Pipeline.process(Pipeline.java:227)
at net.sf.okapi.common.pipeline.Pipeline.process(Pipeline.java:199)
at net.sf.okapi.common.pipelinedriver.PipelineDriver.processBatch(PipelineDriver.java:182)
at net.sf.okapi.applications.tikal.Main.extractFile(Main.java:1626)
at net.sf.okapi.applications.tikal.Main.process(Main.java:1005)
at net.sf.okapi.applications.tikal.Main.main(Main.java:604)

This is due to the behavior of OpenXMLFilterWriter#getSkeletonWriter, which is doing something stateful and returns null at this point.