Double check size to avoid ArrayIndexOutOfBoundsException (netty#…

…9609)

Motivation:

Recycler$Stack.pop will occurs `ArrayIndexOutOfBoundsException` in some race cases, we should double check `size` even after `scavenge` called.

Modifications:

Double check `size` after `scavenge`

Result:

avoid ArrayIndexOutOfBoundsException in `pop`

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

@@ -511,6 +511,10 @@ DefaultHandle<T> pop() {
                     return null;
                 }
                 size = this.size;
+                if (size <= 0) {
+                    // double check, avoid races
+                    return null;
+                }
             }
             size --;
             DefaultHandle ret = elements[size];