Only set cwd when not empty

For existing consumers of libconatiner to not require cwd inside the
libcontainer code.  This can be done at the runc level and is already
evaluated there.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

libcontainer/init_linux.go

@@ -134,8 +134,10 @@ func finalizeNamespace(config *initConfig) error {
 	if err := w.drop(); err != nil {
 		return err
 	}
-	if err := syscall.Chdir(config.Cwd); err != nil {
-		return err
+	if config.Cwd != "" {
+		if err := syscall.Chdir(config.Cwd); err != nil {
+			return err
+		}
 	}
 	return nil
 }