Dynamic port forwarding server closes session after first unsuccessful connection

After first unsuccessful connection attempt through proxy dynamic port forwarding socket gets closed. How to reproduce:

1. Setup dynamic port forwarding
2. Try connecting to unreachable host through proxy

The reason for this is because sshd tries to do graceful channel close on channel that hasn't been opened. After unsuccessful attempt to open forwading channel to unreachable host sshd sends SSH_MSG_CHANNEL_CLOSE to which SSH server responds with error and the whole session is going down (tested with SSH-2.0-OpenSSH_5.3).

I've attached a patch with fix proposal. In my case that fixed the issue.