Fix bugs around uni-coherence invalidates being propogated properly.

Fix bugs around uni-coherence invalidates being propogated properly.

src/mem/bus.cc:
    Make it so that invalidates being sent from the responder up don't call the responder
    but they should also not Panic.
src/mem/packet.hh:
    If we don't have data in the packet, don't call deleteData:
    Example: InvalidateRequests never have data.

--HG--
extra : convert_revision : 18766bc9f3bb4d852ac651d094254d347abd1634