xprops: Null-terminate property reply values

Some of the mutter code using these properties expects them to be
null-terminated whereas xcb does not use null-terminated strings:

http://xcb.freedesktop.org/XcbRationale/

This was in some cases resulting in the WM_CLASS property containing
garbage data which broke application matching, caused the hot-corner and
window-switcher to stop working, or was exposed as text in the UI.

https://bugzilla.gnome.org/show_bug.cgi?id=759658