[FIX] web: prevent useless rpc in one2many

In some cases (for example, some popup dialogs with a form view), the
one2many field is destroyed but the async code still tries to append
a view manager to itself.  The result is that the start method of the
view manager is run, resulting in at least an useless fieldviewget.

addons/web/static/src/js/views/form_relational_widgets.js

@@ -850,7 +850,9 @@ var FieldOne2Many = AbstractManyField.extend({
             });
         });
         $.async_when().done(function () {
-            self.viewmanager.appendTo(self.$el);
+            if (!self.isDestroyed()) {
+                self.viewmanager.appendTo(self.$el);
+            }
         });
         return def;
     },