memberof: check for empty arrays to avoid segfaults
The arrays with members to add or delete may be empty, i.e. have 0
entries. In this case further processing should be skipped to avoid
segfaults later on.
Fixes (hopefully) https://fedorahosted.org/sssd/ticket/2430
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit a1bd8bc666df7fa696523ec8ec1dfe3d79780588)