Fix crash related to ssb-fixtures and index feeds

[staltz]

See https://gist.github.com/arj03/88176e218171e847aa6523681ee92eab

Crash looks like:

Error: found invalid message: This feed is forked. Last known good message was as seq: 428: {
    "previous": "%dvOZI6uz6dJP7Nj8L8Hys4fS3uyHiRNFAji+2XKvApQ=.sha256",
    "sequence": 429,
    "author": "@aAHW4h9g0+m+ItT58gdbi8d4i3JA4HyNnx8+B1BD5m0=.ed25519",
    "timestamp": 1634040904007,
    "hash": "sha256",
    "content": "OlRUI7c85LKbB/Sjv9Fdcm/atxx2fZvXuZBeRNW8wBrsG4cM7pF+/RRUCdIMvkt2jeviJ9yGcWTNiMCXWycqcAmKnU9ImAdUG6BL1N4F14M/EmTK8XMdjynj6V3I82Y/H1398SVXQEBSKrsX9LTs2BxYjjXNdcGcWlQhcCMGEqQUYhYxM0mMOo46UtZcPqs6oFqdMTAvlNr2QzqSw2OCY1jDSe5p9fMlIogHWB4QV5GcPF4UwHKD3ul2LB9jyhf0V0aPOsURmxn/EH9lNN5Y0nBcnCRxARm3fqYWgmesTUOEfi7og1Mvx3LcIXMj9QQeLDDrnRZ7LpXSK8lEHpmLNY9afkMEYqeejAOmf/exayTAlXvWxpwp3+P2KFjPcpWz4Vhqb1VgnrbVTYVkBgmJtylmK3jU3H2hxAZzEg6PrOsH0nkvRgbT157wOk8X0kYLkrmWS+Px8GD5zS54oxBYypNMyW036ljb+KJl2Uo=.box",
    "signature": "uo4KsbIQhY6H+FGxpXG+sueYjzbtRwPIWf//qElngmCSkMMvTihR3g4y9/5p2hUXpGJIy2/W4QrH9fnnrO0cCw==.sig.ed25519"
  }

[staltz]

cc @arj03 just to remember this issue