dbgring: ignore blocks of 0s
dbgring: ignore blocks of 0s

These are more likely to be unused space than explicit "Debug"
packets.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

mirage/ocaml-xenstore-server,djs55/ocaml-xenstore-server,mirage/ocaml-xenstore-server