Ticket 48351 - Fix buffer overflow error when reading url with len 0
    https://fedorahosted.org/389/ticket/48351
    
    Bug Description:  In ldaputil.c it's possible to have url_to_use with a len of 0
    This means we are reading from an undefined area of memory.
    
    Fix Description:  Check len before the smprintf, and if 0, then provide a
    a default of "/" which matches the theoretical behaviour of the format. We also
    have a stronger check to prevent NULL from being passed as a URL to validate.
    
    Author: wibrown
    
    Review by: nhosoi, mreynolds (Thanks!)