fixed incorrect assert (patch #943970)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@27662 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dynload.h

@@ -277,7 +277,7 @@ public:
     {
         Load(libname, flags);
     }
-    ~wxPluginManager() { Unload(); }
+    ~wxPluginManager() { if ( IsLoaded() ) Unload(); }
 
     bool   Load(const wxString &libname, int flags = wxDL_DEFAULT);
     void   Unload();