Fix a crash after creating a constraint.

To reproduce, e.g.:
  * draw a rect;
  * translate the rect;
  * add a pt-on-line constraint between two instances.

src/constraint.cpp

@@ -83,6 +83,7 @@ hConstraint Constraint::AddConstraint(Constraint *c, bool rememberForUndo) {
     if(rememberForUndo) SS.UndoRemember();
 
     SK.constraint.AddAndAssignId(c);
+    c->Generate(&SK.param);
 
     SS.MarkGroupDirty(c->group);
     SK.GetGroup(c->group)->dofCheckOk = false;