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

src/main/java/org/jboss/netty/channel/DefaultChannelFuture.java

@@ -364,6 +364,10 @@ public class DefaultChannelFuture implements ChannelFuture {
     }
 
     public boolean setFailure(Throwable cause) {
+        if (cause == null) {
+            throw new NullPointerException("cause");
+        }
+
         synchronized (this) {
             // Allow only once.
             if (done) {