certmap: make sure eku_oid_list is always allocated
If there are only OIDs in a <EKU> part of a matching rule a NULL pointer
dereference might occur.
Related to https://pagure.io/SSSD/sssd/issue/3508
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>