Add a missing null pointer check in s_write.

I don't know that this can ever be triggered in the current state of
the code, but when I start mucking around with SSH session closing in
the near future, it may be handy to have it.

[originally from svn r10076]

ssh.c

@@ -1762,6 +1762,8 @@ static int s_write(Ssh ssh, void *data, int len)
     if (ssh->logctx)
 	log_packet(ssh->logctx, PKT_OUTGOING, -1, NULL, data, len,
 		   0, NULL, NULL);
+    if (!ssh->s)
+        return 0;
     return sk_write(ssh->s, (char *)data, len);
 }