unix: fix staticcheck issues

Change-Id: If7abd7a743c4359a9780401cec7c874d530ee268
Reviewed-on: https://go-review.googlesource.com/c/sys/+/396674
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/internal/mkmerge/mkmerge_test.go

@@ -410,6 +410,10 @@ const (
 		}
 
 		got, err := filter(inFiles[0].src, commonSet.keepCommon)
+		if err != nil {
+			t.Fatal(err)
+		}
+
 		expected := []byte(mergedFile)
 
 		if !bytes.Equal(got, expected) {