Avoid hanging on painting dashed lines with non-finite coordinates

The dash stroker did not check for inf/nan coordinates.

Fixes: QTBUG-47887
Change-Id: I1e696cd15cc37d8fcb6a464cac3da33c3a8b95c2
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/painting/qstroker.cpp

@@ -1150,6 +1150,8 @@ void QDashStroker::processCurrentSubpath()
 
     QSubpathFlatIterator it(&m_elements, m_dashThreshold);
     qfixed2d prev = it.next();
+    if (!prev.isFinite())
+        return;
 
     bool clipping = !m_clip_rect.isEmpty();
     qfixed2d move_to_pos = prev;
@@ -1165,6 +1167,8 @@ void QDashStroker::processCurrentSubpath()
     bool hasMoveTo = false;
     while (it.hasNext()) {
         QStrokerOps::Element e = it.next();
+        if (!qfixed2d(e).isFinite())
+            continue;
 
         Q_ASSERT(e.isLineTo());
         cline = QLineF(qt_fixed_to_real(prev.x),

src/gui/painting/qstroker_p.h

@@ -104,6 +104,7 @@ struct qfixed2d
     qfixed x;
     qfixed y;
 
+    bool isFinite() { return qIsFinite(x) && qIsFinite(y); }
     bool operator==(const qfixed2d &other) const { return qFuzzyCompare(x, other.x)
                                                        && qFuzzyCompare(y, other.y); }
 };