explicitly clear onDecoded callback

Author: nkzawa

src/main/java/io/socket/client/Manager.java

@@ -507,6 +507,7 @@ private void cleanup() {
 
         On.Handle sub;
         while ((sub = this.subs.poll()) != null) sub.destroy();
+        this.decoder.onDecoded(null);
 
         this.packetBuffer.clear();
         this.encoding = false;