CVE-2020-10704: libcli ldap_message: Add search size limits to ldap_decode

CVE-2020-10704: libcli ldap_message: Add search size limits to ldap_decode

Add search request size limits to ldap_decode calls.

The ldap server uses the smb.conf variable
"ldap max search request size" which defaults to 250Kb.
For cldap the limit is hard coded as 4096.

Credit to OSS-Fuzz

REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20454
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14334

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>