screen: Fix uninitialized rect in myScreenGetXineramaMonitorGeometry

XineramaIsActive function may fail if we disconnect all monitors, so always initialize the rect with the screen size.

It prevents passing uninitialized values to clientReconfigure in case when CLIENT_FLAG_FULLSCREEN_MONITORS flag is set, but Xinerama is not active.

This fixes enormous full screen window size (games run by Proton) causing game or PC to freeze/crash/OOM when monitor is disconnected.