do not crash when a non-active generator asks for the document

svn path=/trunk/KDE/kdegraphics/okular/; revision=787699

core/generator.cpp

@@ -332,7 +332,11 @@ void Generator::signalPixmapRequestDone( PixmapRequest * request )
 const Document * Generator::document() const
 {
     Q_D( const Generator );
-    return d->m_document->m_parent;
+    if ( d->m_document )
+    {
+        return d->m_document->m_parent;
+    }
+    return 0;
 }
 
 void Generator::setFeature( GeneratorFeature feature, bool on )