minor issue mesh_booleans_inputcheck crashes on meshes with unused vertices

[rob-rb]

Iff the e_link is empty in the code. We get an exception on front.

mplate<class M, class V, class E, class P>
CINO_INLINE
bool AbstractPolygonMesh<M,V,E,P>::vert_is_manifold(const uint vid) const
{
for(uint eid : this->adj_v2e(vid))
{
if(!this->edge_is_manifold(eid)) return false;
}

std::vector<uint> e_link = this->vert_edges_link(vid);
std::unordered_set<uint> edge_set(e_link.begin(), e_link.end());

std::queue<uint> q;
q.push(e_link.front());