hw/net: Fix a heap overflow in xlnx.xps-ethernetlite

The .receive callback of xlnx.xps-ethernetlite doesn't check the length
of data before calling memcpy. As a result, the NetClientState object in
heap will be overflowed. All versions of qemu with xlnx.xps-ethernetlite
will be affected.

Reported-by: chaojianhu <chaojianhu@hotmail.com>
Signed-off-by: chaojianhu <chaojianhu@hotmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
