calling vv_range on valence 0 vertices causes a crash

When you call vv_range on a vertexhandle of a vertex with valence 0 OpenMesh will crash as vv_range uses the circulatorRange which uses cvv_end and cvv_begin which are marked as deprecated.

I propose to use cvv_cwend and cvv_cwbegin which are mentioned in the documentation to be used instead of the deprecated one.