ArduPlane: Fix spiralling loiters when no fence return point can be found

ArduPlane/fence.cpp

@@ -92,9 +92,11 @@ void Plane::fence_check()
                     guided_WP_loc.lat = return_point[0];
                     guided_WP_loc.lng = return_point[1];
                 } else {
-                    // should. not. happen.
-                    guided_WP_loc.lat = current_loc.lat;
-                    guided_WP_loc.lng = current_loc.lng;
+                    // When no fence return point is found (ie. no inclusion fence uploaded, but exclusion is)
+                    // we fail to obtain a valid fence return point. In this case, home is considered a safe
+                    // return point.
+                    guided_WP_loc.lat = home.lat;
+                    guided_WP_loc.lng = home.lng;
                 }
             }