Moodle device selection causes error if $USER global is null

If Moodle displays a screen when the $USER global is null, this causes an error because this scenario is not checked for in the code. Only time this happens AFAIK is when a wwwrootmismatch error occurs, but presumably there are other errors.

Debug info: Argument 1 passed to check_user_preferences_loaded() must be an instance of stdClass, null given, called in /fs1/www_root/ajf425/github/lib/moodlelib.php on line 1553 and defined
Stack trace:

line 359 of /lib/setuplib.php: coding_exception thrown
line 1327 of /lib/moodlelib.php: call to default_error_handler()
line 1553 of /lib/moodlelib.php: call to check_user_preferences_loaded()
line 7764 of /lib/moodlelib.php: call to get_user_preferences()
line 534 of /lib/pagelib.php: call to get_user_device_type()
line 615 of /lib/pagelib.php: call to moodle_page->magic_get_devicetypeinuse()
line 1335 of /lib/pagelib.php: call to moodle_page->__get()
line 1257 of /lib/pagelib.php: call to moodle_page->resolve_theme()
line 1244 of /lib/setuplib.php: call to moodle_page->initialise_theme_and_output()
line 2495 of /lib/weblib.php: call to bootstrap_renderer->__call()
line 2495 of /lib/weblib.php: call to bootstrap_renderer->redirect_message()
line 676 of /lib/setuplib.php: call to redirect()
line 656 of /lib/setup.php: call to initialise_fullme()
line 29 of /config.php: call to require_once()
line 31 of /index.php: call to require_once()