Check newconn, not conn, before marking newconn for close

This fixes bug 2757.  It's a bugfix on 85da676108f0d.

changes/bug2757

@@ -0,0 +1,6 @@
+  - Minor bugfixes
+    o Avoid a double-mark-for-free warning when failing to attach a
+      transparent proxy connection.  (We thought we had fixed this in
+      0.2.2.23-alpha, but it turns out our fix was checking the wrong
+      connection.)  Fixes bug 2757; bugfix on 0.1.2.1-alpha (the original
+      bug) and 0.2.2.23-alpha (the incorrect fix).

src/or/connection.c

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