Fix interface-wrapped nil returned by MakeOpenFile

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>

file.go

@@ -111,7 +111,13 @@ func makeWin32File(h syscall.Handle) (*win32File, error) {
 }
 
 func MakeOpenFile(h syscall.Handle) (io.ReadWriteCloser, error) {
-	return makeWin32File(h)
+	// If we return the result of makeWin32File directly, it can result in an
+	// interface-wrapped nil, rather than a nil interface value.
+	f, err := makeWin32File(h)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
 }
 
 // closeHandle closes the resources associated with a Win32 handle