Fix refreshing wxGenericListCtrl with multiple selection

Changing the selection could behave wrongly and result in an assert
failure as the selection anchor could have become invalid, due to the
change of the number of items in the control.

Fix this by invalidating it when this happens.

src/generic/listctrl.cpp

@@ -3730,6 +3730,10 @@ void wxListMainWindow::SetItemCount(long count)
     if ( HasCurrent() && m_current >= (size_t)count )
         ChangeCurrent(count - 1);
 
+    // And do the same thing for the multiple selection anchor.
+    if ( m_anchor != (size_t)-1 && m_anchor >= (size_t)count )
+        m_anchor = count - 1;
+
     m_selStore.SetItemCount(count);
     m_countVirt = count;