Numerous SSH_MSG_CHANNEL_FAILURE messages sent for SSH_MSG_CHANNEL_DATA on unknown channel

Hi in case the SSHD server receives a message for a not existing channel, it sends back a SSH_MSG_CHANNEL_FAILURE.

This behaviour is not described in RFC4254, which, I agree, is unclear about what should be do in this case.

Since the current SSHD implementation send back a SSH_MSG_CHANNEL_FAILURE message (with the local channel id), in our environment, it put the receipient in an inconsistent state. 

In out enviroment happened that the receipient has sent a SSH_MSG_CHANNEL_DATA while SSHD had already closed the channel.

I fully agree that this should not happen (I will open another issue about this) but the SSH_MSG_CHANNEL_FAILURE further complicates things.

The problem can be overcome modifying line 78 of org.apache.sshd.common.session.helpers.DefaultUnknownChannelReferenceHandler

letting

case SshConstants.SSH_MSG_CHANNEL_EXTENDED_DATA:
// Not sure if entirely compliant with RFC4254, but try to stem the flood	
wantReply = false;
break;

default: // do nothing

instead of

case SshConstants.SSH_MSG_CHANNEL_EXTENDED_DATA:
// Not sure if entirely compliant with RFC4254, but try to stem the flood	
wantReply = true;
break;

default: // do nothing