[bugfix] calculation of numTopologies was wrong for dim<0

as the reference elements are used in generic code, the situation can happen, that one tries to access a reference-element of dim-1 (i.e. faces) for a 0d object. This would result in a reference-element for dim=-1, which has no topolgies. We fix this explicitly by checking the dim>=0.

changed milestone to %DUNE 2.7.0

added bug label

@gruenich this MR should not be critical. Can we still have this in 2.7?

Would it be worth to add a test which would fail without your change? If yes, could you write one?

merged

mentioned in commit de4c5e6c