Chore: Address race condition in channel.go

## Overview

This was created from a `TODO` comment:

```go
// TODO (@kyle): There's an obvious race-condition here.
// This is an edge-case, as most-frequently data won't
// be pooled so synchronously, but is definitely possible.
//
// See: https://github.com/pion/sctp/issues/181
```

See also #1066 