ldb:attrib_handlers: use ldb_ascii_toupper() in first loop

ldb:attrib_handlers: use ldb_ascii_toupper() in first loop

In a dotless-I locale, we might meet an 'i' before we meet a byte with
the high bit set, in which case we still want the ldb casefold
comparison.

Many ldb operations will do some case-folding before getting here, so
hitting this might be quite rare even in those locales.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15637

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>