Fix handling of relative URLs starting with "/" in wxHTML

Don't interpret them relatively to the current path, so that an URL like
"/pictures/foo.png" in the file "/webpages/bar.html" is resolved correctly and
not as "/webpages/pictures/foo.png" as it was previously.

See #17148.

src/html/winpars.cpp

@@ -323,7 +323,10 @@ wxFSFile *wxHtmlWinParser::OpenURL(wxHtmlURLType type,
         wxString redirect;
         status = m_windowInterface->OnHTMLOpeningURL(type, myfullurl, &redirect);
         if ( status != wxHTML_REDIRECT )
+        {
+            myurl = myfullurl;
             break;
+        }
 
         myurl = redirect;
     }