Avoid heap-buffer-overflow

[ChangeLog][QCosmeticStroker] Avoid a heap-buffer-overflow found by oss-
fuzz as issue 25243.

Pick-to: 5.12 5.15
Change-Id: I36112f183241679e172ad1ee531e1b929d6f3815
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/painting/qcosmeticstroker.cpp

@@ -101,7 +101,7 @@ struct Dasher {
             offset += stroker->patternLength;
 
         dashIndex = 0;
-        while (offset>= pattern[dashIndex])
+        while (dashIndex < stroker->patternSize - 1 && offset>= pattern[dashIndex])
             ++dashIndex;
 
 //        qDebug() << "   dasher" << offset/64. << reverse << dashIndex;