markup-renderer: use safe fetching of attributes to prevent from errors on malformed html.

rhodecode/lib/markup_renderer.py

@@ -83,12 +83,12 b' def relative_links(html_source, server_p'
         return html_source
 
     for el in doc.cssselect('img, video'):
-        src = el.attrib['src']
+        src = el.attrib.get('src')
         if src:
             el.attrib['src'] = relative_path(src, server_path)
 
     for el in doc.cssselect('a:not(.gfm)'):
-        src = el.attrib['href']
+        src = el.attrib.get('href')
         if src:
             el.attrib['href'] = relative_path(src, server_path)