openpgp: prevent panic for missing self-signature

There can be public keys where an identity has no self-signature. Since
the order of identities as tested against `shouldPreferIdentity` is
unstable (due to being based on a map iteration), such a situation leads
to a panic in a certain percentage of runs.

Avoid the panic by preferring the potential new identity. This can still
lead to unusable results, but they will returned as a proper error,
allowing an application to handle it.

openpgp/keys.go

@@ -99,6 +99,10 @@ func shouldPreferIdentity(existingId, potentialNewId *Identity) bool {
 		return false
 	}
 
+	if existingId.SelfSignature == nil {
+		return true
+	}
+
 	if (existingId.SelfSignature.IsPrimaryId != nil && *existingId.SelfSignature.IsPrimaryId &&
 		!(potentialNewId.SelfSignature.IsPrimaryId != nil && *potentialNewId.SelfSignature.IsPrimaryId)) {
 		return false