If socket is about to be closed do not add to [read|write]_set.

If NSG_CLOSE_IMMEDIATELY is on, then the socket will be closed,
which means that (if the socket was added to either read_set or
write_set) the subsequent select will in turn be performed on a
socket that has been closed.

Standard socket implementations seem to ignore a descriptor
referring to a closed socket but LWIP
(http://savannah.nongnu.org/projects/lwip/) crashes. So better
not to add to the sets.

mongoose.c

@@ -1024,6 +1024,9 @@ time_t ns_mgr_poll(struct ns_mgr *mgr, int milli) {
     if (!(conn->flags & (NSF_LISTENING | NSF_CONNECTING))) {
       ns_call(conn, NS_POLL, &current_time);
     }
+    if (conn->flags & NSF_CLOSE_IMMEDIATELY) {
+      ns_close_conn(conn);
+    } else {
       if (!(conn->flags & NSF_WANT_WRITE)) {
         //DBG(("%p read_set", conn));
         ns_add_to_set(conn->sock, &read_set, &max_fd);
@@ -1034,8 +1037,6 @@ time_t ns_mgr_poll(struct ns_mgr *mgr, int milli) {
         //DBG(("%p write_set", conn));
         ns_add_to_set(conn->sock, &write_set, &max_fd);
       }
-    if (conn->flags & NSF_CLOSE_IMMEDIATELY) {
-      ns_close_conn(conn);
     }
   }