calendarspec: encode that it's OK to store µs in 'int's

src/shared/calendarspec.c

@@ -30,6 +30,9 @@
  * linked compenents anyway. */
 #define CALENDARSPEC_COMPONENTS_MAX 240
 
+/* Let's make sure that the microsecond component is safe to be stored in an 'int' */
+assert_cc(INT_MAX >= USEC_PER_SEC);
+
 static void chain_free(CalendarComponent *c) {
         CalendarComponent *n;