Fix possible segfault with debug library shenanigans

local info = debug.getinfo(0)
for i=1, math.huge do
  local newInfo = debug.getinfo(i)
  if not newInfo then
    info.func()
    print("you have done little sorcery UwU")
    return
  end
  info = newInfo
end

src/lua.cpp

@@ -631,6 +631,7 @@ static void doREPL (lua_State *L) {
 */
 static int pmain (lua_State *L) {
   int argc = (int)lua_tointeger(L, 1);
+  luaL_checktype(L, 2, LUA_TLIGHTUSERDATA);  /* [Pluto] let's not deref any nullptrs if the user is being funny with the 'debug' library */
   char **argv = (char **)lua_touserdata(L, 2);
   int script;
   int args = collectargs(argv, &script);