autofs: fix invalid header 'number of entries' in packet
    
    https://fedorahosted.org/sssd/ticket/1739
    
    Pointer to packet body may change while filling packet with autofs
    mount points. As a consequence, we sometimes wrote the number of
    entries into invalid body and we recieved an arbitrary number
    on the client side.
    
    If the number was 0, there were some skipped entries. If the number
    was greater than 0, everything worked correctly, because we iterate
    through the cached entries until we reach packet length - we don't
    compare to the number.