tdf#89460 test pChangeTrack before using it

Change-Id: Ic02351c448929daa7d4c5a9a00df9842c2c5aa91
Reviewed-on: https://gerrit.libreoffice.org/14553Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/ui/docshell/docsh4.cxx

@@ -586,8 +586,11 @@ void ScDocShell::Execute( SfxRequest& rReq )
 
                     if ( bDo )
                     {
-                        if ( pChangeTrack->IsProtected() )
-                            bDo = ExecuteChangeProtectionDialog( NULL );
+                        if (pChangeTrack)
+                        {
+                            if ( pChangeTrack->IsProtected() )
+                                bDo = ExecuteChangeProtectionDialog( NULL );
+                        }
                         if ( bDo )
                         {
                             rDoc.EndChangeTracking();