katzenpost/catshadow: client.go; 8 LoC

Found a possible issue in [katzenpost/catshadow](https://www.github.com/katzenpost/catshadow) at [client.go](https://github.com/katzenpost/catshadow/blob/4f2d32f4193b1122b7f57cdf181caf90417f03bf/client.go#L229-L236)

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.

> range-loop variable contact used in defer or goroutine at line 233

[Click here to see the code in its original context.](https://github.com/katzenpost/catshadow/blob/4f2d32f4193b1122b7f57cdf181caf90417f03bf/client.go#L229-L236)

Click here to show the 8 line(s) of Go which triggered the analyzer.

```go
for _, contact := range c.contacts {
if !contact.IsPending {
if _, err := contact.outbound.Peek(); err == nil {
// prod worker to start draining contact outbound queue
defer func() { c.opCh <- &opRetransmit{contact: contact} }()
}
}
}

```

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: 4f2d32f4193b1122b7f57cdf181caf90417f03bf