bitrise-io/codesigndoc: osxkeychain/osxkeychain.go; 16 LoC

Found a possible issue in [bitrise-io/codesigndoc](https://www.github.com/bitrise-io/codesigndoc) at [osxkeychain/osxkeychain.go](https://github.com/bitrise-io/codesigndoc/blob/53c14b032dd4a8655427fa93aa129358bc0b2b13/osxkeychain/osxkeychain.go#L148-L163)

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 aIdentityRef is reassigned at line 160

[Click here to see the code in its original context.](https://github.com/bitrise-io/codesigndoc/blob/53c14b032dd4a8655427fa93aa129358bc0b2b13/osxkeychain/osxkeychain.go#L148-L163)

Click here to show the 16 line(s) of Go which triggered the analyzer.

```go
for _, aIdentityRef := range foundIdentityRefs {
cert, err := GetCertificateDataFromIdentityRef(aIdentityRef.KeychainRef)
if err != nil {
return nil, fmt.Errorf("Failed to read certificate data, error: %s", err)
}

if err := certificateutil.CheckValidity(*cert); err != nil {
log.Warnf("Certificate is not valid, skipping: %s", err)
continue
}

if latestIdentityRef == nil || cert.NotAfter.After(latestCertificate.NotAfter) {
latestIdentityRef = &aIdentityRef
latestCertificate = *cert
}
}

```

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commit ID: 53c14b032dd4a8655427fa93aa129358bc0b2b13