Prevent warnings about unconstructed attached ROMs

If attached ROM dataspaces are not valid after update(), code that uses
these ROMs produces

  void Genode::Volatile_object< <template-parameter-1-1> >::_check_constructed() const [with MT = Genode::Attached_dataspace]: Deref_unconstructed_object

repos/gems/src/server/wm/main.cc

@@ -70,6 +70,8 @@ struct Wm::Main
 	{
 		try {
 			focus_rom.update();
+			if (!focus_rom.is_valid())
+				return;
 
 			unsigned long win_id = 0;
 
@@ -94,6 +96,8 @@ struct Wm::Main
 	{
 		try {
 			resize_request_rom.update();
+			if (!resize_request_rom.is_valid())
+				return;
 
 			char const * const node_type = "window";
 

repos/os/src/app/status_bar/main.cc

@@ -228,6 +228,8 @@ void Main::handle_focus(unsigned)
 {
 	/* fetch new content of the focus ROM module */
 	focus_ds.update();
+	if (!focus_ds.is_valid())
+		return;
 
 	/* reset status-bar properties */
 	label       = Label();

repos/os/src/server/rom_filter/input_rom_registry.h

@@ -77,6 +77,8 @@ class Rom_filter::Input_rom_registry
 				void _handle_rom_changed(unsigned)
 				{
 					_rom_ds.update();
+					if (!_rom_ds.is_valid())
+						return;
 
 					try {
 						_top_level = Xml_node(_rom_ds.local_addr<char>());