calendar: make freeing a calendar spec object deal fine with NULL

In order to make object destruction easier (in particular in combination
with _cleanup_) we usually make destructors deal with NULL objects as
NOPs. Change the calendar spec destructor to follow the same scheme.

src/core/load-fragment.c

@@ -1366,7 +1366,6 @@ int config_parse_timer(const char *unit,
 
         v = new0(TimerValue, 1);
         if (!v) {
-                if (c)
                 calendar_spec_free(c);
                 return log_oom();
         }

src/shared/calendarspec.c

@@ -35,7 +35,9 @@ static void free_chain(CalendarComponent *c) {
 }
 
 void calendar_spec_free(CalendarSpec *c) {
-        assert(c);
+
+        if (!c)
+                return;
 
         free_chain(c->year);
         free_chain(c->month);