Set prev to null when setting cursor to head in scavengeSome.

Motivation:

`scavengeSome()` has a corner case: when setting `cursor` to `head`, `this.prev` may point to the tail of the  `WeakOrderQueue` linked list. Then it's possible that the following while loop will link the tail to the head, and cause endless loop.

I made a reproducer in 36522e7b72 . The unit test will just run forever. Unfortunately, I cannot change it to a unit test because it needs to add some codes to `scavengeSome` to control the execution flow.

Modification:

Set `prev` to null when setting `cursor` to `head` in `scavengeSome`

Result:

Fixes #6153.

common/src/main/java/io/netty/util/Recycler.java

@@ -479,16 +479,19 @@ public abstract class Recycler<T> {
         }
 
         boolean scavengeSome() {
+            WeakOrderQueue prev;
             WeakOrderQueue cursor = this.cursor;
             if (cursor == null) {
+                prev = null;
                 cursor = head;
                 if (cursor == null) {
                     return false;
                 }
+            } else {
+                prev = this.prev;
             }
 
             boolean success = false;
-            WeakOrderQueue prev = this.prev;
             do {
                 if (cursor.transfer(this)) {
                     success = true;