CVE-2015-5330: strupper_talloc_n_handle(): properly count characters

When a codepoint eats more than one byte we really want to know,
especially if the string is not NUL terminated.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11599

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Pair-programmed-with: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
