Minor fix plus new assertion to catch similar bugs.

Minor fix plus new assertion to catch similar bugs.

src/cpu/memtest/memtest.cc:
    Need to set packet source field so that response from cache
    doesn't run into assertion failure when copying source to dest.
src/mem/packet.hh:
    Copy source field when copying packets.
    Assert that source is valid before copying it to dest
    when turning packets around.

--HG--
extra : convert_revision : 09e3cfda424aa89fe170e21e955b295746832bf8