Fix double-mark bug when failing to init transparent connection

Fixes part of bug 2279.  Bugfix on 0.1.2.1-alpha.

changes/bug2279

@@ -0,0 +1,5 @@
+  o Minor bugfixes
+    - Avoid a double mark-for-free warning when failing to attach a
+      transparent proxy connection.  Fixes bug 2279.  Bugfix on
+      Tor 0.1.2.1 alpha.
+

src/or/connection.c

@@ -1178,7 +1178,8 @@ connection_handle_listener_read(connection_t *conn, int new_type)
   }
 
   if (connection_init_accepted_conn(newconn, conn->type) < 0) {
-    connection_mark_for_close(newconn);
+    if (! conn->marked_for_close)
+      connection_mark_for_close(newconn);
     return 0;
   }
   return 0;