past EOM and fail with "short read error". This check was part of
traverse.c::bread(), and was removed when that function was migrated
to rcache.c::rawread() as part of the "read cache" functionality.
This should fix the problem with dump barfing on "short read" when "-r 0",
but I'm still debugging the problem where dump gets to a point and
"does nothing" when the read cache is being used.