New macro 'pushvfstring'

Helps to ensure that 'luaO_pushvfstring' is being called correctly,
with an error check after closing the vararg list with 'va_end'.

lapi.c

@@ -593,12 +593,8 @@ LUA_API const char *lua_pushfstring (lua_State *L, const char *fmt, ...) {
   const char *ret;
   va_list argp;
   lua_lock(L);
-  va_start(argp, fmt);
-  ret = luaO_pushvfstring(L, fmt, argp);
-  va_end(argp);
+  pushvfstring(L, argp, fmt, ret);
   luaC_checkGC(L);
-  if (ret == NULL)  /* error? */
-    luaD_throw(L, LUA_ERRMEM);
   lua_unlock(L);
   return ret;
 }