call mg_tcpip_free from mg_mgr_free()

test/fuzz.c

@@ -145,9 +145,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     mg_iobuf_add(&c->recv, 0, data, size);
     c->pfn(c, MG_EV_READ, NULL); // manually invoke protocol event handler
 
-    mg_mgr_free(&mgr);
     free(pkt);
-    mg_tcpip_free(&mif);
   }
 
   return 0;