only set the missed output argument when it is not NULL

@@ -59,9 +59,9 @@ TR_timerGet(TR_Timer this, unsigned long * missed)
 	if (_missed) {
 		this->sec  += _missed * this->timeout * sec_factor / nan_factor;
 		this->nsec += _missed * this->timeout * sec_factor % nan_factor;
-		*missed = _missed;
+		if (missed) *missed = _missed;
 	} else {
-		*missed = 0;
+		if (missed) *missed = 0;
 	}
 
 	return this->timeout - (elapsed % this->timeout);