companyzero/zkc: ratchet/ratchet.go; 17 LoC

Found a possible issue in [companyzero/zkc](https://www.github.com/companyzero/zkc) at [ratchet/ratchet.go](https://github.com/companyzero/zkc/blob/943787798f6c61e42fa971ae724b63498a84985f/ratchet/ratchet.go#L551-L567)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

> reference to headerKey was used in a composite literal at line 564

[Click here to see the code in its original context.](https://github.com/companyzero/zkc/blob/943787798f6c61e42fa971ae724b63498a84985f/ratchet/ratchet.go#L551-L567)

Click here to show the 17 line(s) of Go which triggered the analyzer.

```go
for headerKey, messageKeys := range r.saved {
keys := make([]disk.RatchetState_SavedKeys_MessageKey, 0, len(messageKeys))
for messageNum, savedKey := range messageKeys {
if now.Sub(savedKey.timestamp) > lifetime {
continue
}
keys = append(keys, disk.RatchetState_SavedKeys_MessageKey{
Num: messageNum,
Key: dup32(&savedKey.key),
CreationTime: savedKey.timestamp.Unix(),
})
}
s.SavedKeys = append(s.SavedKeys, disk.RatchetState_SavedKeys{
HeaderKey: dup32(&headerKey),
MessageKeys: keys,
})
}

```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: 943787798f6c61e42fa971ae724b63498a84985f