Fixed splitting of self-connected (half-)edges.

(Half)Edge splits would create inconsistent mesh state when splitting a self-connecting (half)edge. Previously the following would fail:

Mesh m;
auto v0 = m.vertices().add();
auto v1 = m.vertices().add();
auto e = m.edges().add_or_get(v0,v1);
m.edges().split(e);
m.assert_consistency();

Similar for halfedges. Now fixed.

Note: also fixed an annoying warning on gcc about missing parentheses.