bug in ts_rank_cd

There is a bug in ts_rank_cd. It does not correctly give rank when the
query lexeme is the first one in the tsvector.

Example:

select ts_rank_cd(to_tsvector('english', 'abc sdd'),
plainto_tsquery('english', 'abc'));
 ts_rank_cd
------------
          0

select ts_rank_cd(to_tsvector('english', 'bcg abc sdd'),
plainto_tsquery('english', 'abc'));
 ts_rank_cd
------------
        0.1

The problem is that the Cover finding algorithm ignores the lexeme at
the 0th position, I have attached a patch which fixes it. After the
patch the result is fine.

select ts_rank_cd(to_tsvector('english', 'abc sdd'), plainto_tsquery(
'english', 'abc'));
 ts_rank_cd
------------
        0.1