Zero Logger behaves like Nop logger. (rs#606)

log.go

@@ -496,6 +496,9 @@ func (l *Logger) newEvent(level Level, done func(string)) *Event {
 
 // should returns true if the log event should be logged.
 func (l *Logger) should(lvl Level) bool {
+	if l.w == nil {
+		return false
+	}
 	if lvl < l.level || lvl < GlobalLevel() {
 		return false
 	}