ActsEvent: Add missing protection for ConstMultiTrajectory::referenceSurface_impl
If a client forgets to call setSurfaces on a ConstMultiTrajectory, then we currently get a crash when calling e.g. hasReferenceSurface on the track states (in my case because this returns a surface at istate 22, but m_surfaces is empty).
Fixed by adding the same protection as is in MutableMultiTrajectory::referenceSurface_impl