modify user deleted in AD crashes winsync
    
    https://fedorahosted.org/freeipa/ticket/1382
    crash in winsync if replaying a MOD and user does not exist in AD
    If the AD entry is deleted before the deletion can be synced back to IPA,
    and in the meantime an operation is performed on the corresponding
    entry in IPA that should be synced to AD, winsync attempts to get the
    AD entry and it is empty.  This just means the operation will not go
    through, and the entry will be deleted when the sync from AD happens.
    The IPA winsync plugin needs to handle the case when the ad_entry
    is NULL.