Initialize FirstPtBlock.next to make static analysis happy

Could in theory be triggered with a 0 line spanning elipsis.

Pick-to: 6.1
Change-Id: I2166ee354d2f7488e1fcddfcb8c949c8ca2452fe
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/painting/qregion.cpp

@@ -3569,6 +3569,7 @@ static QRegionPrivate *PolygonRegion(const QPoint *Pts, int Count, int rule)
     int fixWAET = false;
     POINTBLOCK FirstPtBlock, *curPtBlock; /* PtBlock buffers    */
     FirstPtBlock.pts = reinterpret_cast<QPoint *>(FirstPtBlock.data);
+    FirstPtBlock.next = nullptr;
     POINTBLOCK *tmpPtBlock;
     int numFullPtBlocks = 0;