CID 1432024 (#1 of 1): Uninitialized scalar variable (UNINIT)
2. uninit_use_in_call: Using uninitialized value rect.x when calling
calendar_arrow_rectangle.

Add a default case to the switch which will bail out with
g_assert_not_reached(), which should reassure Coverity that the method
is always called with a valid value that is handled in the switch.