Fix a bug where ChannelFuture.setFailure(null) doesn't fail

Motivation:

We forgot to do a null check on the cause parameter of
ChannelFuture.setFailure(cause)

Modifications:

Add a null check

Result:

Fixed issue: #2728

common/src/main/java/io/netty/util/concurrent/DefaultPromise.java

@@ -471,6 +471,10 @@ public class DefaultPromise<V> extends AbstractFuture<V> implements Promise<V> {
     }
 
     private boolean setFailure0(Throwable cause) {
+        if (cause == null) {
+            throw new NullPointerException("cause");
+        }
+
         if (isDone()) {
             return false;
         }