Fix convex hull crash

By applying dimforge#168 .

src/shape/convex_polygon.rs

@@ -32,6 +32,9 @@ impl ConvexPolygon {
     /// Convexity of the input polyline is not checked.
     /// Returns `None` if all points form an almost flat line.
     pub fn from_convex_polyline(mut points: Vec<Point<Real>>) -> Option<Self> {
+        if points.is_empty() {
+            return None;
+        }
         let eps = ComplexField::sqrt(crate::math::DEFAULT_EPSILON);
         let mut normals = Vec::with_capacity(points.len());
 
@@ -64,7 +67,7 @@ impl ConvexPolygon {
         points.truncate(new_length);
         normals.truncate(new_length);
 
-        if points.len() != 0 {
+        if points.len() > 2 {
             Some(ConvexPolygon { points, normals })
         } else {
             None