ipfs pin ls fails on old repo

#### Version information:

```
$ ipfs version --all
go-ipfs version: 0.4.17-
Repo version: 7
System version: amd64/linux
Golang version: go1.10.3
```

#### Type:

Bug / Support Issue

#### Description:

I booted up an old ipfs box, tried to check my pins using whatever old versio of ipfs it had:

```
$ ipfs pin ls
Error: The block referred to by 'QmfKGWG9hUFFE8wyMLKszYk4p9oZywpGmHrxWEUgumhX4h' was not a valid merkledag node
```

It looked like a bug, since I didn't recall having any reason to think the data was corrupt. So I upgraded ipfs to this current version and migrated the repo from 6 to 7. Unfortunately I still get an error:

```
$ ipfs pin ls
Error: failed to decode Protocol Buffers: incorrectly formatted merkledag node: unmarshal failed. proto: illegal wireType 7
```

which is even more cryptic!

Any hints on how to recover what remains from these pins?