Make it legal to have a conn with INVALID_SOCKET

E.g. a timer-only connection

PUBLISHED_FROM=e8d4d2b6e5e0dfa8e8f987b2ded6b973c39e2919

mongoose.c

@@ -3721,15 +3721,15 @@ time_t mg_mgr_poll(struct mg_mgr *mgr, int timeout_ms) {
   mg_add_to_set(mgr->ctl[1], &read_set, &max_fd);
 #endif
 
+  /*
+   * Note: it is ok to have connections with sock == INVALID_SOCKET in the list,
+   * e.g. timer-only "connections".
+   */
   min_timer = 0;
   for (nc = mgr->active_connections, num_fds = 0; nc != NULL; nc = tmp) {
     tmp = nc->next;
 
-    if (nc->sock == INVALID_SOCKET) {
-      mg_mgr_handle_conn(nc, 0, now);
-      continue;
-    }
-
+    if (nc->sock != INVALID_SOCKET) {
       num_fds++;
 
       if (!(nc->flags & MG_F_WANT_WRITE) &&
@@ -3743,6 +3743,7 @@ time_t mg_mgr_poll(struct mg_mgr *mgr, int timeout_ms) {
         mg_add_to_set(nc->sock, &write_set, &max_fd);
         mg_add_to_set(nc->sock, &err_set, &max_fd);
       }
+    }
 
     if (nc->ev_timer_time > 0) {
       if (num_timers == 0 || nc->ev_timer_time < min_timer) {
@@ -3781,6 +3782,7 @@ time_t mg_mgr_poll(struct mg_mgr *mgr, int timeout_ms) {
 
   for (nc = mgr->active_connections; nc != NULL; nc = tmp) {
     int fd_flags = 0;
+    if (nc->sock != INVALID_SOCKET) {
       if (num_ev > 0) {
         fd_flags = (FD_ISSET(nc->sock, &read_set) ? _MG_F_FD_CAN_READ : 0) |
                    (FD_ISSET(nc->sock, &write_set) ? _MG_F_FD_CAN_WRITE : 0) |
@@ -3797,6 +3799,7 @@ time_t mg_mgr_poll(struct mg_mgr *mgr, int timeout_ms) {
       /* With LWIP socket emulation layer, we don't get write events */
       fd_flags |= _MG_F_FD_CAN_WRITE;
 #endif
+    }
     tmp = nc->next;
     mg_mgr_handle_conn(nc, fd_flags, now);
   }