os: improve Readdir and Readdirnames test coverage

R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/46450043

src/pkg/os/os_test.go

@@ -252,11 +252,25 @@ func TestReaddirnames(t *testing.T) {
 	testReaddirnames(sysdir.name, sysdir.files, t)
 }
 
+func TestReaddirnamesNilFile(t *testing.T) {
+	var f *File
+	if fi, err := f.Readdirnames(1); fi != nil || err != ErrInvalid {
+		t.Errorf("Readdirnames should fail when f is nil: %v, %v", fi, err)
+	}
+}
+
 func TestReaddir(t *testing.T) {
 	testReaddir(".", dot, t)
 	testReaddir(sysdir.name, sysdir.files, t)
 }
 
+func TestReaddirNilFile(t *testing.T) {
+	var f *File
+	if fi, err := f.Readdir(1); fi != nil || err != ErrInvalid {
+		t.Errorf("Readdir should fail when f is nil: %v, %v", fi, err)
+	}
+}
+
 // Read the directory one entry at a time.
 func smallReaddirnames(file *File, length int, t *testing.T) []string {
 	names := make([]string, length)