CVE-2015-5330: ldb_dn_escape_value: use known string length, not strlen()

ldb_dn_escape_internal() reports the number of bytes it copied, so
lets use that number, rather than using strlen() and hoping a zero got
in the right place.

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>
