USN-4888-2: ldb vulnerabilities

USN-4888-1 fixed several vulnerabilities in ldb. This update provides
the corresponding update for Ubuntu 14.04 ESM.

Original advisory details:

Douglas Bagnall discovered that ldb, when used with Samba, incorrectly
handled certain LDAP attributes. A remote attacker could possibly use this
issue to cause the LDAP server to crash, resulting in a denial of service.
(CVE-2021-20277)

Douglas Bagnall discovered that ldb, when used with Samba, incorrectly
handled certain DN strings. A remote attacker could use this issue to
cause the LDAP server to crash, resulting in a denial of service, or
possibly execute arbitrary code. (CVE-2020-27840)