Fix badAtom issues introduced by behavior changes between Xlib and xcb.

Fix badAtom issues introduced by behavior changes between Xlib and xcb.

In Xlib, the 'length' variable gets updated with the the actual number
of 8-bit, 16-bit, or 32-bit items stored in the returned data, but xcb
returns the actual number of bytes read through xcb_get_property_value_length,
therefore the logic of calculating offset was broken.

Task-number: QTBUG-26709
Change-Id: I04de3b5c5631cfaf9b3c2c3d4513be73c569f61f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>