Avoid overflows when incrementing parameters in C

Any C function can receive maxinteger as an integer argument, and
therefore cannot increment it without some care (e.g., doing unsigned
arithmetic as the core does).

ltablib.c

@@ -59,8 +59,9 @@ static void checktab (lua_State *L, int arg, int what) {
 
 
 static int tinsert (lua_State *L) {
-  lua_Integer e = aux_getn(L, 1, TAB_RW) + 1;  /* first empty element */
   lua_Integer pos;  /* where to insert new element */
+  lua_Integer e = aux_getn(L, 1, TAB_RW);
+  e = luaL_intop(+, e, 1);  /* first empty element */
   switch (lua_gettop(L)) {
     case 2: {  /* called with only 2 arguments */
       pos = e;  /* insert new element at the end */