return immediately after processing TaskbarCreated message instead of asserting (the accepted part of patch 1835507)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@50294 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/taskbar.cpp

@@ -276,6 +276,7 @@ long wxTaskBarIcon::WindowProc(unsigned int msg,
     {
         m_iconAdded = false;
         SetIcon(m_icon, m_strTooltip);
+        return 0;
     }
 
     // this function should only be called for gs_msg(Restart)Taskbar messages