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'.

lcode.c

@@ -43,11 +43,7 @@ static int codesJ (FuncState *fs, OpCode o, int sj, int k);
 l_noret luaK_semerror (LexState *ls, const char *fmt, ...) {
   const char *msg;
   va_list argp;
-  va_start(argp, fmt);
-  msg = luaO_pushvfstring(ls->L, fmt, argp);
-  va_end(argp);
-  if (msg == NULL)  /* error? */
-    luaD_throw(ls->L, LUA_ERRMEM);
+  pushvfstring(ls->L, argp, fmt, msg);
   ls->t.token = 0;  /* remove "near <token>" from final message */
   luaX_syntaxerror(ls, msg);
 }