The unit tests SetCrystalLocationTest, LoadEventNexusTest were failing due to
some detectors in the input nexus file not having a corresponding workspace
index. This showed up as the pixelID_to_wi_vector vector having
std::numeric_limits<size_t>::max() as the wi value
I've added check that wi<numEventLists before trying to retrieve the spectrum
for wi which is the same check that's done elsewhere in ProcessBankData.cpp

If the file BSS_11841_event.nxs had been loaded with compression on
this would have caused a crash even before my fix to the sort order