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>