If validateascii is passed a string containing only non-zero 7bit

values, then the loop with exit with i == len, and the following
test will access beyond the end of the array.

So add an extra test to fix this.

Found by Marcus Meissner <meissner@novell.com>.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>