avoiding crashes on osx during app shutdown

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55518 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/string.h

@@ -671,6 +671,10 @@ private:
       // profiling seems to show a small but consistent gain if we use this
       // simple loop instead of starting from the last used element (there are
       // a lot of misses in this function...)
+      // during destruction tls calls may return NULL, thus return NULL 
+      // immediately
+      if ( GetCacheBegin() == NULL )
+        return NULL;
       for ( Cache::Element *c = GetCacheBegin(); c != GetCacheEnd(); c++ )
       {
           if ( c->str == this )