speakup: i18n: Switch to kmemdup_nul() in spk_msg_set()

Use kmemdup_nul() helper instead of open-coding to
simplify the code in spk_msg_set().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210406034434.442251-1-yangyingliang@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>