dont NUL terminate _NET_WM_NAME

dont NUL terminate _NET_WM_NAME

Reported by Kernc, thanks!

"This makes a particular program that uses libwnck [1] fail after:

    Wnck-WARNING **: Property _NET_WM_NAME contained invalid UTF-8

in this code [2] because the returned string contains a '\0' and the
documentation for g_utf8_validate() [3] explicitly states that when
string length is provided, no nul bytes are allowed."

It is not entirely clear it is incorrect, other WM's seem to not
NUL terminate it either though.