lk2nd: device: menu: Check if FB is actually initialized

There is no fbcon_config if the display failed to initialize or if the
device is running without display (e.g. on DragonBoard 410c). Add a
check in display_fastboot_menu() that avoids crashing if that is the
case.

lk2nd/device/menu/menu.c

@@ -140,6 +140,9 @@ void display_fastboot_menu(void)
 	int y, y_menu, old_scale, incr;
 	unsigned int sel = 0, i;
 
+	if (!fb)
+		return;
+
 	/*
 	 * Make sure the specified line lenght fits on the screen.
 	 */