Fix undefined behavior in thread.c

This was doing pointer addition where i could be >= LUV_THREAD_MAXNUM_ARG and then subtracting afterwards. By doing the subtraction first, the number is properly bounded within 0..LUV_THREAD_MAXNUM_ARG

src/thread.c

@@ -72,7 +72,7 @@ static int luv_thread_arg_set(lua_State* L, luv_thread_arg_t* args, int idx, int
   args->flags = flags;
   while (i <= top && i < LUV_THREAD_MAXNUM_ARG + idx)
   {
-    luv_val_t *arg = args->argv + i - idx;
+    luv_val_t *arg = args->argv + (i - idx);
     arg->type = lua_type(L, i);
     arg->ref[0] = arg->ref[1] = LUA_NOREF;
     switch (arg->type)