Fix crash in client get_packet() call

[pthedinger]

There is a bug where the mii_ethernet_rt_mac.xc can crash when handling the client get_packet() call on the line:

  info.src_ifnum = buf->src_port;

The only case that it seems this would be possible is when there are both high and low priority clients of the MAC and the following occurs:

1. A packet is received for a low priority client and the client is notified.
2. Many more packets are received for the high priority client and the buffers start to fill.
3. The low priority packets are dropped to make room.
4. The client comes to collect the packet. The pointer is NULL at that point, but is not being checked.

It is an easy fix, to check for NULL before using buf, and to return desc.type = ETH_NO_DATA; in that case. However, it would be best to have a test case to demonstrate this issue before fixing it.