fixed memory corruption in wxHTML when parsing '&;' in the markup

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55389 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/html/htmlpars.cpp

@@ -557,6 +557,9 @@ wxChar wxHtmlEntitiesParser::GetEntityChar(const wxString& entity) const
 {
     unsigned code = 0;
 
+    if (entity.empty())
+      return 0; // invalid entity reference
+
     if (entity[0] == wxT('#'))
     {
         // NB: parsed value is a number, so it's OK to use wx_str(), internal