The root cause (and that still needs fixing) is that the OOXML signature
export fails to start same-document references with a "#" character.
OTOH, even if that happens, it's better to throw uno::RuntimeException
in UriBindingHelper::OpenInputStream() to avoid std::terminate().

Change-Id: I9c9c211de36fb0aeb2c33f62b094c9f4d9c85b3d