Fix #145560: Invalid curve flag set in legacy curve pen tool

Instead of just disabling the cyclic bit of the flag, the code
reset the whole flag to -2 which doesn't really make sense.
It cause a problem because the "custom knots" bit was set
but there weren't any allocated custom knots.

Pull Request: https://projects.blender.org/blender/blender/pulls/145580

source/blender/editors/curve/editcurve_pen.cc

@@ -1125,7 +1125,7 @@ static void extrude_points_from_selected_vertices(const ViewContext *vc,
                                      new_last_nu,
                                      new_last_nu->bezt ? (const void *)new_last_nu->bezt :
                                                          (const void *)new_last_nu->bp);
-      new_last_nu->flagu = ~CU_NURB_CYCLIC;
+      new_last_nu->flagu &= ~CU_NURB_CYCLIC;
     }
   }