os: fail if Open("") is called on windows

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5432071

src/pkg/os/file_windows.go

@@ -89,6 +89,9 @@ func openDir(name string) (file *File, err error) {
 // methods on the returned File can be used for I/O.
 // It returns the File and an error, if any.
 func OpenFile(name string, flag int, perm uint32) (file *File, err error) {
+	if name == "" {
+		return nil, &PathError{"open", name, syscall.ENOENT}
+	}
 	// TODO(brainman): not sure about my logic of assuming it is dir first, then fall back to file
 	r, e := openDir(name)
 	if e == nil {

src/pkg/os/os_test.go

@@ -901,6 +901,14 @@ func TestOpenError(t *testing.T) {
 	}
 }
 
+func TestOpenNoName(t *testing.T) {
+	f, err := Open("")
+	if err == nil {
+		t.Fatal(`Open("") succeeded`)
+		f.Close()
+	}
+}
+
 func run(t *testing.T, cmd []string) string {
 	// Run /bin/hostname and collect output.
 	r, w, err := Pipe()