XCB: prevent a fp division by zero

For certain devices vci->resolution is zero, which causes a SIGFPE
if FE_INVALID exceptions are enabled. Try to prevent that.

Task-number: QTBUG-42717
Change-Id: I388735f5dfb6218496787dbb74cf0c0f43cc928f
Reviewed-by: Alexander Volkov <a.volkov@rusbitech.ru>
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>