MuonHough: Avoid use-after-free error.

A MuonLayerHough::Maximum object has a pointer to a MuonLayerHough object. The latter lives in a temporary State object. The Maximum objects, however, can end up being entered in a vector and recorded in SG. In such a case, the State objects can then be deleted. leaving dangling pointers in the Maximum objects. This has not been observed to cause crashes; instead, it just occasionally gives wrong results.