Follow up fix in Lua lvm.c

The "Return hook" fix introduced a 64-bit compiler warning about
integer truncation.

It looks safe to cast away the warning.  And that's reinforced
by the fact that other places in the file use `cast_int()` for
the exact same expression.

lua/src/lvm.c

@@ -757,7 +757,7 @@ void luaV_execute (lua_State *L) {
       vmcasenb(OP_RETURN,
         int b = GETARG_B(i);
         if (cl->p->sizep > 0) luaF_close(L, base);
-        b = luaD_poscall(L, ra, (b != 0 ? b - 1 : L->top - ra));
+        b = luaD_poscall(L, ra, (b != 0 ? b - 1 : cast_int(L->top - ra)));
         if (!(ci->callstatus & CIST_REENTRY))  /* 'ci' still the called one */
           return;  /* external invocation: return */
         else {  /* invocation via reentry: continue execution */