Wrap correct error on unix.GetsockoptUcred failure

Wrap ucredErr which is set by unix.GetsockoptUcred, not the nil err.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

unixcreds_linux.go

@@ -50,7 +50,7 @@ func (fn UnixCredentialsFunc) Handshake(ctx context.Context, conn net.Conn) (net
 	}
 
 	if ucredErr != nil {
-		return nil, nil, fmt.Errorf("ttrpc.UnixCredentialsFunc: failed to retrieve socket peer credentials: %w", err)
+		return nil, nil, fmt.Errorf("ttrpc.UnixCredentialsFunc: failed to retrieve socket peer credentials: %w", ucredErr)
 	}
 
 	if err := fn(ucred); err != nil {