[Positioning] Fix RP generation with >3 APs

When selecting the 2 reference APs, skip the APs that do not have (yet)
an associated reference point.

owlps-positioning/src/stock.cc

@@ -520,6 +520,10 @@ void Stock::regenerate_reference_points()
                   continue ;
                 const Point3D &ref_coord =
                   ref->second.get_coordinates() ;
+                // Skip the AP if the associated reference point does
+                // not exist yet:
+                if (! reference_point_exists(ref_coord))
+                  continue ;
                 double angle =
                   rx_coord.angle(current_point, ref_coord) ;
                 pair<double,