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 <[e-mail]>
Signed-off-by: Yang Yingliang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/accessibility/speakup/i18n.c

···
 	if ((index < MSG_FIRST_INDEX) || (index >= MSG_LAST_INDEX))
 		return -EINVAL;
 
-	newstr = kmalloc(length + 1, GFP_KERNEL);
+	newstr = kmemdup_nul(text, length, GFP_KERNEL);
 	if (!newstr)
 		return -ENOMEM;
 
-	memcpy(newstr, text, length);
-	newstr[length] = '\0';
 	if (index >= MSG_FORMATTED_START &&
 	    index <= MSG_FORMATTED_END &&
 	    !fmt_validate(speakup_default_msgs[index], newstr)) {