Fix a bug in ChannelOutboundBuffer.nioBuffers()

Related issue: #2717, #2710, #2704, #2693

Motivation:

When ChannelOutboundBuffer.nioBuffers() iterates over the linked list of
entries, it is not supposed to visit unflushed entries, but it does.

Modifications:

- Make sure ChannelOutboundBuffer.nioBuffers() stops the iteration before
  it visits an unflushed entry
- Add isFlushedEntry() to reduce the chance of the similar mistakes

Result:

Another regression is gone.

transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java

@@ -356,7 +356,7 @@ public final class ChannelOutboundBuffer {
         final InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
         ByteBuffer[] nioBuffers = NIO_BUFFERS.get(threadLocalMap);
         Entry entry = flushedEntry;
-        while (entry != null && entry.msg instanceof ByteBuf) {
+        while (isFlushedEntry(entry) && entry.msg instanceof ByteBuf) {
             if (!entry.cancelled) {
                 ByteBuf buf = (ByteBuf) entry.msg;
                 final int readerIndex = buf.readerIndex();
@@ -586,7 +586,11 @@ public final class ChannelOutboundBuffer {
                 }
             }
             entry = entry.next;
-        } while (entry != null && entry != unflushedEntry);
+        } while (isFlushedEntry(entry));
+    }
+
+    private boolean isFlushedEntry(Entry e) {
+        return e != null && e != unflushedEntry;
     }
 
     public interface MessageProcessor {