Protect STS digitizers against unphysical MCPoint times
With Geant4 in fairsoft jun19, sometimes unphysically high MCPoint times (> 10^9 ns from event start) appear. This leads to an excess of the time limit storable in CbmStsDigi (2^31 ns) in simulations event-by-event or with flexible time slices. With this MR, such MCPoints are discarded for digitization.