[netty#2363] Correctly null out SelectionKey[] when selectAgain

Motivation:

The prefix fix of netty#2363 did not correctly handle the case when selectAgain is true and so missed to null out entries.

Modifications:

Move the i++ from end of loop to beginning of loop

Result:

Entries in the array will be null out so allow to have these GC'ed once the Channel close

Author: jiafu1115

transport/src/main/java/io/netty/channel/nio/NioEventLoop.java

@@ -475,11 +475,11 @@ private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
                 // null out entries in the array to allow to have it GC'ed once the Channel close
                 // See https://github.com/netty/netty/issues/2363
                 for (;;) {
+                    i++;
                     if (selectedKeys[i] == null) {
                         break;
                     }
                     selectedKeys[i] = null;
-                    i++;
                 }
 
                 selectAgain();