libertas: potential oops in debugfs

If we do a zero size allocation then it will oops.  Also we can't be
sure the user passes us a NUL terminated string so I've added a
terminator.

This code can only be triggered by root.

Reported-by: Nico Golde <nico@ngolde.de>
Reported-by: Fabian Yamaguchi <fabs@goesec.de>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
static ssize_t lbs_highsnr_write(struct file *file, const char __user *userbuf,
				 size_t count, loff_t *ppos)
{
	return lbs_threshold_write(TLV_TYPE_SNR_HIGH, CMD_SUBSCRIBE_SNR_HIGH,
				   file, userbuf, count, ppos);
}

static ssize_t lbs_highsnr_write(struct file *file, const char __user *userbuf,
				 size_t count, loff_t *ppos)
{
	return lbs_threshold_write(TLV_TYPE_SNR_HIGH, CMD_SUBSCRIBE_SNR_HIGH,
				   file, userbuf, count, ppos);
}
