Fix null dereference in ssh_unthrottle.

The backend_unthrottle function gets called when the backlog on stdout
clears, and it's possible for that to happen _after_ the SSH backend
has terminated the connection and freed all its protocol modules (e.g.
if a protocol error occurred on the network while data was still
waiting to be written to stdout). So ssh_unthrottle should check that
ssh->cl still exists before calling any method of it.

ssh.c

@@ -1013,7 +1013,8 @@ static void ssh_unthrottle(Backend *be, int bufsize)
 {
     Ssh *ssh = container_of(be, Ssh, backend);
 
-    ssh_stdout_unthrottle(ssh->cl, bufsize);
+    if (ssh->cl)
+        ssh_stdout_unthrottle(ssh->cl, bufsize);
 }
 
 static bool ssh_connected(Backend *be)