duplicated some code from 4943~4953, but that code has it's own problem, it forget to reset *bytes results to
larger frame->datalen could read beyond the buffer, and it also makes stats not accurate. But if we reset *bytes
at that place, then later the switch_vb_put_packet has problem because it depends that *bytes. this patch should
fix the datalen at least buf still leaves duplicated code and inaccurate stats.