Ensure the selection is in range.

There are some cases when in the process of closing wxAuiMDIChildFrames that this method will be called while the current selection is out of range.

src/aui/tabmdi.cpp

@@ -820,7 +820,7 @@ int wxAuiMDIClientWindow::SetSelection(size_t nPage)
 wxAuiMDIChildFrame* wxAuiMDIClientWindow::GetActiveChild()
 {
     const int sel = GetSelection();
-    if ( sel == wxNOT_FOUND )
+    if ( sel == wxNOT_FOUND || sel >= GetPageCount() )
         return NULL;
 
     return wxStaticCast(GetPage(sel), wxAuiMDIChildFrame);