venti: Session.read(): Handle reads for 0-score/any type locally

vac archives may have the zero-score (da39...) with any type.
I've observed type VtDirType + 3, for a single 720 MiB file, for
example.

The reference venti server will not find zero-score blocks with
type != 0; and the reference client does not send reads for
zero-score blocks regardless of type (see libventi/client.c:
vtreadpacket).

Implement the same logic in venti.b.

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