Commit ef2668ba (PPTX import: fix
missing document metadata, 2014-08-28) implemented metadata import for
the PPTX filter, but in case the metadata stream is not an existing one,
then OHierarchyHolder_Impl::GetListPathFromString() invoked by
OStorage::openStreamElementByHierarchicalName() throws.

The bugdoc is generated by a 3rd-party tool that always starts the
stream path with a slash, and MSO seems to just ignore that: so let's do
the same to be able to open the document.

Change-Id: I6c0715adeb19b9055669f6a45055415dd2c44e28