[fix] bing: parsing result; check to see if the element contains links

This patch is to hardening the parsing of the bing response:

1. To fix [2087] check if the selected result item contains a link, otherwise
   skip result item and continue in the result loop.  Increment the result
   pointer when a result has been added / the enumerate that counts for skipped
   items is no longer valid when result items are skipped.

   To test the bugfix use:   ``!bi :all cerbot``

2. Limit the XPath selection of result items to direct children nodes (list
   items ``li``) of the ordered list (``ol``).

   To test the selector use: ``!bi :en pontiac aztek wiki``

   .. in the result list you should find the wikipedia entry on top,
   compare [2068]

[2087] https://github.com/searxng/searxng/issues/2087
[2068] https://github.com/searxng/searxng/issues/2068