CVE-2020-10704: libcli ldap: test recursion depth in ldap_decode_filter_tree

CVE-2020-10704: libcli ldap: test recursion depth in ldap_decode_filter_tree

Add tests to check that ASN.1 ldap requests with deeply nested elements
are rejected.  Previously there was no check on the on the depth of
nesting and excessive nesting could cause a stack overflow.

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>