Fix Segmentation fault in the Release build

Backtrace:
    LVStyleSheet::apply(ldomNode const *, css_style_rec_tag *)                                                                                     0x55555567bc28
    setNodeStyle(ldomNode *, LVFastRef<css_style_rec_tag>, LVProtectedFastRef<LVFont>)                                                             0x5555556d2d23
    ldomNode::initNodeStyle()                                                                                                                      0x555555640954
    updateStyleDataRecursive(ldomNode *)                                                                                                           0x555555640e50
    updateStyleDataRecursive(ldomNode *)                                                                                                           0x555555640ef0
    updateStyleDataRecursive(ldomNode *)                                                                                                           0x555555640ef0
    updateStyleDataRecursive(ldomNode *)                                                                                                           0x555555640ef0
    ldomDocument::render(LVRendPageList *, LVDocViewCallback *, int, int, bool, int, LVProtectedFastRef<LVFont>, int, LVFastRef<CRPropAccessor>)   0x555555646771
    LVDocView::Render(int, int, LVRendPageList *)                                                                                                  0x5555556ae862
    LVDocView::checkRender()                                                                                                                       0x5555556aea50
    LVDocView::updateBookMarksRanges()                                                                                                             0x5555556b2fa6
    LVDocView::propsApply(LVFastRef<CRPropAccessor>)                                                                                               0x5555556c7815
    CR3View::loadSettings(QString)                                                                                                                 0x5555555ce64e
    MainWindow::MainWindow(QWidget *)                                                                                                              0x5555555da585
    main                                                                                                                                           0x5555555b928c

crengine/src/lvstsheet.cpp

@@ -967,6 +967,7 @@ bool LVCssSelectorRule::check( const ldomNode * & node )
             for (;;)
             {
                 node = node->getParentNode();
+                if (!node) return false;
                 if (node->isNull())
                     return false;
                 if (node->getNodeId() == _id)