Fix to make g++-10.2 happy

g++-10.2 fails to compile qregion.cpp in RelWithDebInfo configuration,
with error message:
qregion.cpp:3617:12: error: ‘ET.EdgeTable::ymax’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
g++ is right here, for Count==1, 'ET' is not initialized but used.
This patch fixes this by Q_ASSUMEing Count > 1.

Pick-to: 5.15
Change-Id: I3b9f1f58de9f3811c60640a08334487e3f8f2b23
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/gui/painting/qregion.cpp

@@ -3572,6 +3572,8 @@ static QRegionPrivate *PolygonRegion(const QPoint *Pts, int Count, int rule)
     POINTBLOCK *tmpPtBlock;
     int numFullPtBlocks = 0;
 
+    Q_ASSUME(Count > 1);
+
     region = new QRegionPrivate;
 
         /* special case a rectangle */