NULL ptr dereference in LAN91xDxe

[Mark Salter]

I tried adding LAN91xDxe support to Foundation model and ran
into a NULL ptr dereference of DstAddr in SnpTransmit():

  // Check for the nature of the frame
  if (DstAddr->Addr[0] == 0xFF) {
    LanDriver->Stats.TxBroadcastFrames += 1;
  } else if ((DstAddr->Addr[0] & 0x1) == 1) {
    LanDriver->Stats.TxMulticastFrames += 1;
  } else {
    LanDriver->Stats.TxUnicastFrames += 1;
  }

DstAddr is passed into SnpTransmit and is an optional argument.
Not sure why this wasn't seen on FVP model. Patch attached, but
it may not be the proper fix.

[Leif Lindholm (leif-lindholm)]

Steve, have a look at how this pans out once ARM's Ethernet drivers go upstream. Should be in time for 2014.05.