Fix #1539 - proper timer cleanup

src/net.c

@@ -230,6 +230,7 @@ void mg_mgr_free(struct mg_mgr *mgr) {
   struct mg_connection *c;
   struct mg_timer *tmp, *t = mgr->timers;
   while (t != NULL) tmp = t->next, free(t), t = tmp;
+  mgr->timers = NULL;  // Important. Next call to poll won't touch timers
   for (c = mgr->conns; c != NULL; c = c->next) c->is_closing = 1;
   mg_mgr_poll(mgr, 0);
 #if MG_ARCH == MG_ARCH_FREERTOS_TCP