Fix #1710 - free MIP in mg_mgr_free()

mongoose.c

@@ -3499,6 +3499,7 @@ void mg_mgr_free(struct mg_mgr *mgr) {
 #if MG_ENABLE_EPOLL
   if (mgr->epoll_fd >= 0) close(mgr->epoll_fd), mgr->epoll_fd = -1;
 #endif
+  free(mgr->priv);
 }
 
 void mg_mgr_init(struct mg_mgr *mgr) {