[#2666] Fix possible NPE when try to fullfill connect ChannelPromise

Motivation:

Because of a missing return statement we may produce a NPE when try to fullfill the connect ChannelPromise when it was fullfilled before.

Modification:

Add missing return statement.

Result:

No more NPE.

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollSocketChannel.java

@@ -496,6 +496,7 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
         private void fulfillConnectPromise(ChannelPromise promise, Throwable cause) {
             if (promise == null) {
                 // Closed via cancellation and the promise has been notified already.
+                return;
             }
 
             // Use tryFailure() instead of setFailure() to avoid the race against cancel().

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

@@ -268,6 +268,7 @@ public abstract class AbstractNioChannel extends AbstractChannel {
         private void fulfillConnectPromise(ChannelPromise promise, Throwable cause) {
             if (promise == null) {
                 // Closed via cancellation and the promise has been notified already.
+                return;
             }
 
             // Use tryFailure() instead of setFailure() to avoid the race against cancel().