functions may return a CFStringRef that is null, so we make this conversion always return an empty string (crashes otherwise)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39670 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/mac/corefoundation/cfstring.cpp

@@ -666,6 +666,9 @@ void wxMacCFStringHolder::Assign( const wxString &st , wxFontEncoding encoding )
 
 wxString wxMacCFStringHolder::AsString(wxFontEncoding encoding)
 {
+    if ( m_cfs == NULL )
+        return wxEmptyString ;
+    
     Size cflen = CFStringGetLength( m_cfs )  ;
     size_t noChars ;
     wxChar* buf = NULL ;