Fix receive on UDP connections

Should only read on outgoing or listening "connections"

PUBLISHED_FROM=e84ff26b899a5f29ddef29ad6c5ba1d18e76e4ad

mongoose.c

@@ -3786,7 +3786,10 @@ time_t mg_mgr_poll(struct mg_mgr *mgr, int timeout_ms) {
     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_flags = (FD_ISSET(nc->sock, &read_set) &&
+                            (!(nc->flags & MG_F_UDP) || nc->listener == NULL)
+                        ? _MG_F_FD_CAN_READ
+                        : 0) |
                    (FD_ISSET(nc->sock, &write_set) ? _MG_F_FD_CAN_WRITE : 0) |
                    (FD_ISSET(nc->sock, &err_set) ? _MG_F_FD_ERROR : 0);
       }