cmd/cover: check error

The variable was assigned but not used, as caught by a tool
written by gorden.klaus@gmail.com.

Change-Id: I84ca3d00896287a35561bd122a0cf64212854a86
Reviewed-on: https://go-review.googlesource.com/10610
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/cover/cover_test.go

@@ -50,6 +50,9 @@ func TestCover(t *testing.T) {
 		lines[i] = bytes.Replace(line, []byte("LINE"), []byte(fmt.Sprint(i+1)), -1)
 	}
 	err = ioutil.WriteFile(coverInput, bytes.Join(lines, []byte("\n")), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	// defer removal of test_line.go
 	if !debug {