API Read Call Panics

```
[dnelson@sulaco keybase-chatbot]$ keybase -v
keybase version 1.0.39-20180119172947+d29f565ed
```

When calling the API, a panic occurs.

The first works, the second causes a panic:
```
{"method": "read", "params": {"options": {"conversation_id": "00006f986cf0c653670c2704e8e694c08bfe9936<24 digits omitted>"}}}

{"method": "read", "params": {"options": {"conversation_id": "0000a3a0d371f5163fa894c31986aa63d2fddd63<24 digits omitted>"}}}
```

The panic:
```
[dnelson@sulaco keybase-chatbot]$ keybase chat api
{"method": "read", "params": {"options": {"conversation_id": "0000a3a0d371f5163fa894c31986aa63d2fddd63<24 digits omitted>"}}}
panic: wrong case accessed

goroutine 1 [running]:
github.com/keybase/client/go/protocol/chat1.MessageUnboxed.Valid(0x3, 0x0, 0x0, 0xc42019f1e0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/protocol/chat1/local.go:2290 +0x9d
github.com/keybase/client/go/client.(*chatServiceHandler).ReadV1(0xc42008a0e0, 0x225c1e0, 0xc420088020, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/chat_svc_handler.go:178 +0x83d
github.com/keybase/client/go/client.(*ChatAPI).ReadV1(0xc4202d48c0, 0x225c1e0, 0xc420088020, 0x0, 0x0, 0x0, 0xc4202a8130, 0x4, 0x0, 0xc4201fe000, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/chat_api_handler.go:293 +0x299
github.com/keybase/client/go/client.(*ChatAPIVersionHandler).handleV1(0xc4202ca490, 0x225c1e0, 0xc420088020, 0x0, 0x0, 0x0, 0xc4202a8130, 0x4, 0x0, 0xc4201fe000, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/chat_api_version_handler.go:36 +0x308
github.com/keybase/client/go/client.(*ChatAPIVersionHandler).handle(0xc4202ca490, 0x225c1e0, 0xc420088020, 0x0, 0x0, 0x0, 0xc4202a8130, 0x4, 0x0, 0xc4201fe000, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/chat_api_version_handler.go:25 +0xb3
github.com/keybase/client/go/client.(*cmdAPI).decode(0xc42016dc48, 0x225c1e0, 0xc420088020, 0x224c0a0, 0xc42008a000, 0x224c0e0, 0xc42008a008, 0x2248b20, 0xc4202ca490, 0xc420302800, ...)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/json_api_common.go:197 +0x161
github.com/keybase/client/go/client.(*cmdAPI).runHandler(0xc42016dc48, 0x2248b20, 0xc4202ca490, 0x0, 0x0)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/json_api_common.go:181 +0xed
github.com/keybase/client/go/client.(*CmdChatAPI).Run(0xc42016dc40, 0xc4202c61c0, 0x2368310)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/client/cmd_chat_api.go:31 +0x102
main.mainInner(0xc4202c2380, 0x0, 0x0)
/root/build/gopaths/amd64/src/github.com/keybase/client/go/keybase/main.go:159 +0x71b
main.main()
/root/build/gopaths/amd64/src/github.com/keybase/client/go/keybase/main.go:64 +0xa6
```

The channel which panics on read:
```
{'id': '0000a3a0d371f5163fa894c31986aa63d2fddd63<24 digits omitted>', 'channel': {'name': '', 'public': False, 'members_type': 'team', 'topic_type': 'chat', 'topic_name': 'general'}, 'unread': False, 'active_at': 1516824884, 'active_at_ms': 1516824884014}
```

The one which works fine:
```
{'id': '00006f986cf0c653670c2704e8e694c08bfe9936<24 digits omitted>', 'channel': {'name': '', 'public': False, 'members_type': 'team', 'topic_type': 'chat', 'topic_name': 'general'}, 'unread': False, 'active_at': 1516818495, 'active_at_ms': 1516818495377}
```

The first channel name contains numbers and an underscore, the second is all lowercase ascii.

Any thoughts? I'm at a loss.

Start by reproducing the second read request with `keybase chat api`, then inspect `go/protocol/chat1/local.go` at `MessageUnboxed.Valid` and the caller in `go/client/chat_svc_handler.go`. Compare the failing and working channel responses and trace the `wrong case accessed` path; done means the failing API call no longer panics and returns an appropriate result.

Written by the indexing model from the issue text.