Error object cannot be nil

Lua will change a nil as error object to a string message, so that it
never reports an error with nil as the error object.

ldebug.c

@@ -844,7 +844,9 @@ l_noret luaG_runerror (lua_State *L, const char *fmt, ...) {
   va_start(argp, fmt);
   msg = luaO_pushvfstring(L, fmt, argp);  /* format message */
   va_end(argp);
-  if (msg != NULL && isLua(ci)) {  /* Lua function? (and no error) */
+  if (msg == NULL)  /* no memory to format message? */
+    luaD_throw(L, LUA_ERRMEM);
+  else if (isLua(ci)) {  /* Lua function? */
     /* add source:line information */
     luaG_addinfo(L, msg, ci_func(ci)->p->source, getcurrentline(ci));
     setobjs2s(L, L->top.p - 2, L->top.p - 1);  /* remove 'msg' */