Fix potential leak in socks client

PUBLISHED_FROM=2cb4449d2f36d59db2b0dc37c46b35de6a461810

mongoose.c

@@ -4209,6 +4209,7 @@ static void mg_socks_if_free(struct mg_iface *iface) {
   LOG(LL_DEBUG, ("%p", iface));
   if (d != NULL) {
     socks_if_disband(d);
+    mbuf_free(&d->tmp);
     MG_FREE(d->proxy_addr);
     MG_FREE(d);
     iface->data = NULL;