Fix crash when no key bindings are available

Change-Id: I89693a01bd3e89f6d4000901c01accf88b453691
Reviewed-on: https://gerrit.libreoffice.org/61232
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

vcl/qt5/Qt5AccessibleWidget.cxx

@@ -684,6 +684,9 @@ QStringList Qt5AccessibleWidget::keyBindingsForAction(const QString& actionName)
     Reference<XAccessibleKeyBinding> xKeyBinding
         = xAccessibleAction->getAccessibleActionKeyBinding(index);
 
+    if (!xKeyBinding.is())
+        return keyBindings;
+
     int count = xKeyBinding->getAccessibleKeyBindingCount();
     for (int i = 0; i < count; i++)
     {