refactor(fold): avoid coverity false-positive

Also add some more argument checks.

src/nvim/lua/stdlib.c

@@ -549,19 +549,21 @@ static int nlua_iconv(lua_State *lstate)
 static int nlua_foldupdate(lua_State *lstate)
 {
  handle_T window = (handle_T)luaL_checkinteger(lstate, 1);
- Error err = ERROR_INIT;
- win_T *win = find_window_by_handle(window, &err);
- if (ERROR_SET(&err)) {
- nlua_push_errstr(lstate, err.msg);
- api_clear_error(&err);
- lua_error(lstate);
- return 0;
+ win_T *win = handle_get_window(window);
+ if (!win) {
+ return luaL_error(lstate, "invalid window");
+ }
+ // input is zero-based end-exclusive range
+ linenr_T top = (linenr_T)luaL_checkinteger(lstate, 2) + 1;
+ if (top < 1 || top > win->w_buffer->b_ml.ml_line_count) {
+ return luaL_error(lstate, "invalid top");
+ }
+ linenr_T bot = (linenr_T)luaL_checkinteger(lstate, 3);
+ if (top > bot) {
+ return luaL_error(lstate, "invalid bot");
  }
 
- linenr_T start = (linenr_T)luaL_checkinteger(lstate, 2);
- linenr_T end = (linenr_T)luaL_checkinteger(lstate, 3);
-
- foldUpdate(win, start + 1, end);
+ foldUpdate(win, top, bot);
 
  return 0;
 }