$USER->set_account_preferences() always saves preferences to the
usr_account_preference table as well as to $USER.  Adding a test for
the user's id means that this function can be safely called on a
logged-out user.

Change-Id: Ieab78c5fbf30647dbb5dce10e960afb260c8dd1f
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>