broken integrity-check-index command

[icristescu]

see https://gitlab.com/tezos/tezos/-/issues/4475

to reproduce in irmin:

❯ dune exec -- ./test/irmin-tezos/irmin_fsck.exe integrity-check-index minimal_store/context
>> Beginning index checking with parameters: { log_size = 2500000 }
Checking index  191.3 KiB 00:00 [----------------------------------------------------------------------------]   0%
irmin-fsck: internal error, uncaught exception:
            Pack_error: "Invalid_argument"

[metanivek]

This has been fixed.