Yet another possible segfault path in the backends fixed. I don't

_believe_ I'm still finding these. I have no idea what I was
thinking when I wrote this stuff.

[originally from svn r1162]

ssh.c

@@ -1478,8 +1478,10 @@ static int ssh_closing(Plug plug, char *error_msg, int error_code,
 		       int calling_back)
 {
     ssh_state = SSH_STATE_CLOSED;
-    sk_close(s);
-    s = NULL;
+    if (s) {
+        sk_close(s);
+        s = NULL;
+    }
     if (error_msg) {
 	/* A socket error has occurred. */
 	connection_fatal(error_msg);