lib/unixfd: Fix data race in unit test.

Change-Id: Ic1d6640fe7da4d6b46f12a5fb26c17fdf67a7ae5
diff --git a/lib/unixfd/unixfd_test.go b/lib/unixfd/unixfd_test.go
index 152efa8..74a2c65 100644
--- a/lib/unixfd/unixfd_test.go
+++ b/lib/unixfd/unixfd_test.go
@@ -142,7 +142,7 @@
 	done := make(chan struct{})
 	buf := make([]byte, 10)
 	go func() {
-		saddr, n, err = ReadConnection(server, buf)
+		saddr, n, readErr = ReadConnection(server, buf)
 		close(done)
 	}()
 	caddr, err := SendConnection(uclient.(*net.UnixConn), []byte("hello"), true)