Fix nullptr dereference with invalid SVG

Fixes: QTBUG-117944
Pick-to: 6.5 6.2
Change-Id: I9059dc28c750fc0585f1fb982152b211c323c6cd
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
(cherry picked from commit edc8ca7)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/svg/qsvghandler.cpp

@@ -3606,6 +3606,8 @@ void QSvgHandler::init()
 
 static bool detectCycles(const QSvgNode *node, QList<const QSvgUse *> active = {})
 {
+    if (Q_UNLIKELY(!node))
+        return false;
     switch (node->type()) {
     case QSvgNode::DOC:
     case QSvgNode::G: