memberof: Don't allocate on a NULL context
https://fedorahosted.org/sssd/ticket/2959
In case no previous delete operation occured, the del_ctx->muops pointer we
allocate the diff structure was would be NULL, effectivelly leaking the
diff array during the memberof processing.
Allocating on del_ctx is safer as that pointer is always allocated and
prevents the leak.
Reviewed-by: Pavel Březina <pbrezina@redhat.com>