Call XdmcpFill with properly initialized length
    
    This is just like commit 86ac166294f0fc8f662031167066acf1928cf139,
    but in a different place in the code.
    
    Spotted by mariodebian <mariodebian@gmail.com>
    (cherry picked from commit f7a7158efcaa3e113a02dd0f0bfdf22a253928e5)