fix crash in the sidebar code

The check should be for xPanel and not xPanels.

See
http://crashreport.libreoffice.org/stats/signature/sfx2::sidebar::Panel::GetTitleBar()

Change-Id: I745d6d8abbfc68e86ed812460faa551867ddec43
Reviewed-on: https://gerrit.libreoffice.org/34560
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

sd/source/ui/sidebar/SlideBackground.cxx

@@ -298,7 +298,7 @@ void SlideBackground::SetPanelTitle( const OUString& rTitle )
         return;
 
     Reference<ui::XPanel> xPanel ( xPanels->getByName("SlideBackgroundPanel"), uno::UNO_QUERY);
-    if ( !xPanels.is() )
+    if ( !xPanel.is() )
         return;
 
     xPanel->setTitle( rTitle );