Avoid useless cast on call luaS_newlstr

[Ranier Vilela <ranier.vf@...>]

Maybe Lua never has to convert such large numbers.

But to satisfy luaS_newlstr, better pass the correct type?

LUAI_FUNC TString *luaS_newlstr (lua_State *L, const char *str, size_t l);

diff --git a/lobject.c b/lobject.c
index f8ea917a..42144a1c 100644
--- a/lobject.c
+++ b/lobject.c
@@ -373,7 +373,7 @@ static int tostringbuff (TValue *obj, char *buff) {
 */
 void luaO_tostring (lua_State *L, TValue *obj) {
   char buff[MAXNUMBER2STR];
-  int len = tostringbuff(obj, buff);
+  size_t len = tostringbuff(obj, buff);
   setsvalue(L, obj, luaS_newlstr(L, buff, len));
 }