lib/nfs/Glue: add assertion

src/lib/nfs/Glue.cxx

@@ -52,6 +52,7 @@ nfs_finish()
 NfsConnection &
 nfs_get_connection(const char *server, const char *export_name)
 {
+	assert(in_use > 0);
 	assert(io_thread_inside());
 
 	return nfs_glue->GetConnection(server, export_name);