Nothing in slip.c calls buffer_init for the first packet. We don't
normally notice this because userv-ipif _both_ prints a confirmation
END byte right away, _and_ bookends packets with ENDs.
But this should be fixed. Otherwise we fail an assertion when we try
to prepend things to the first data packet.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>