hlandauf/btcnode: mempool.go; 10 LoC

Found a possible issue in [hlandauf/btcnode](https://www.github.com/hlandauf/btcnode) at [mempool.go](https://github.com/hlandauf/btcnode/blob/e141647cdc2a8b8d013905b58d8ab06280c10d4f/mempool.go#L477-L486)

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 txHash is reassigned at line 479

[Click here to see the code in its original context.](https://github.com/hlandauf/btcnode/blob/e141647cdc2a8b8d013905b58d8ab06280c10d4f/mempool.go#L477-L486)

Click here to show the 10 line(s) of Go which triggered the analyzer.

```go
for txHash := range mp.orphans {
if foundHash == nil {
foundHash = &txHash
}
txHashNum := btcchain.ShaHashToBig(&txHash)
if txHashNum.Cmp(randHashNum) > 0 {
foundHash = &txHash
break
}
}

```

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See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: e141647cdc2a8b8d013905b58d8ab06280c10d4f