don't crash when the document contents can't be converted to the target encoding (#10064)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@56216 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/xml/xml.cpp

@@ -759,6 +759,8 @@ inline static void OutputString(wxOutputStream& stream, const wxString& str,
     wxUnusedVar(convMem);
 
     const wxWX2MBbuf buf(str.mb_str(*(convFile ? convFile : &wxConvUTF8)));
+    if ( !buf )
+        return;
     stream.Write((const char*)buf, strlen((const char*)buf));
 #else // !wxUSE_UNICODE
     if ( convFile && convMem )