refactor: simplify aucmd_restbuf usage #39879

Problem:
`aucmd_restbuf` must be guarded in case `aucmd_prepbuf` wasn't called.

Solution:
Update `aucmd_restbuf` to be a no-op if `aucmd_prepbuf` wasn't called.
This requires `aco` to be zero-initialized.

src/nvim/lua/stdlib.c

@@ -639,7 +639,7 @@ static int nlua_with(lua_State *L)
 
   Error err = ERROR_INIT;
   TRY_WRAP(&err, {
-    aco_save_T aco;
+    aco_save_T aco = { 0 };
     win_execute_T win_execute_args;
 
     if (win) {