Dont compare short with bool (minetest#4963)

Fixes a compiler warning on MSVC

src/script/lua_api/l_util.cpp

@@ -398,7 +398,8 @@ int ModApiUtil::l_get_dir_list(lua_State *L)
 {
  NO_MAP_LOCK_REQUIRED;
  const char *path = luaL_checkstring(L, 1);
- short is_dir = lua_isboolean(L, 2) ? lua_toboolean(L, 2) : -1;
+ bool list_all = !lua_isboolean(L, 2); // if its not a boolean list all
+ bool list_dirs = lua_toboolean(L, 2); // true: list dirs, false: list files
 
  CHECK_SECURE_PATH(L, path, false);
 
@@ -408,7 +409,7 @@ int ModApiUtil::l_get_dir_list(lua_State *L)
  lua_newtable(L);
 
  for (size_t i = 0; i < list.size(); i++) {
- if (is_dir == -1 || is_dir == list[i].dir) {
+ if (list_all || list_dirs == list[i].dir) {
  lua_pushstring(L, list[i].name.c_str());
  lua_rawseti(L, -2, ++index);
  }