ScreenPool: relax assertion in handleScreenAdded

When a new monitor is connected, the existing monitor might emit
geometryChanged before qGuiApp emits screenAdded, but the new screen is
already in qApp->screens(), so m_sizeSortedScreens still doesn't have
the new screen at that point, which will make handleScreenAdded called
twice.

SENTRY: PLASMA-WORKSPACE-29H
BUG: 483432
FIXED-IN: 6.0.4


(cherry picked from commit 0692d762)