Calendar: Add default case to appease Coverity

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.

gtk/gtkcalendar.c

@@ -1273,6 +1273,9 @@ calendar_arrow_rectangle (GtkCalendar  *calendar,
         rect->x = (allocation.width - padding.left - padding.right
                    - 3 - priv->arrow_width);
       break;
+
+    default:
+      g_assert_not_reached ();
     }
 
   rect->x += padding.left;