Fix a race conditions

Two accesses to a loggers were not protected by a logger's mutex

logger.go

@@ -28,11 +28,17 @@ func NewLogger(name string) (l *Logger) {
 
 // AddBackend add a new Backend to the logger. All set backends are kept.
 func (l *Logger) AddBackend(b Backend) {
+	l.Lock()
+	defer l.Unlock()
+
 	l.backends = append(l.backends, b)
 }
 
 // SetBackend sets the backend list to the logger. Any existing backend will be lost.
 func (l *Logger) SetBackend(b ...Backend) {
+	l.Lock()
+	defer l.Unlock()
+
 	l.backends = b
 }