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

@@ -1366,8 +1366,7 @@ int config_parse_timer(const char *unit,
 
         v = new0(TimerValue, 1);
         if (!v) {
-                if (c)
-                        calendar_spec_free(c);
+                calendar_spec_free(c);
                 return log_oom();
         }