CVE-2015-3223: lib: ldb: Use memmem binary search, not strstr text search.

Values might have embedded zeros.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11325

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
