Guard against crash in QQuickPdfSearchModel::setDocument()

If the document is null, ignore it.

Change-Id: I3cebd049fb5d16d0064dddf00183f231019ef03c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/pdf/quick/qquickpdfsearchmodel.cpp

@@ -67,7 +67,7 @@ QQuickPdfDocument *QQuickPdfSearchModel::document() const
 
 void QQuickPdfSearchModel::setDocument(QQuickPdfDocument *document)
 {
-    if (document == m_quickDocument)
+    if (document == m_quickDocument || !document)
         return;
 
     m_quickDocument = document;