small bug in hlCover

I think there is a slight bug in hlCover function in wparser_def.c

If there is only one query item and that is the first word in the text,
then hlCover does not returns any cover. This is evident in this example
when ts_headline only generates the min_words:

testdb=# select ts_headline('1 2 3 4 5 6 7 8 9 10','1'::tsquery,
'MinWords=5');
   ts_headline
------------------
 <b>1</b> 2 3 4 5
(1 row)

The problem is that *q is initialized to 0 which is a legitimate value
for a cover. So I have attached a patch that fixes it and after applying
the patch here is the result.

testdb=# select ts_headline('1 2 3 4 5 6 7 8 9 10','1'::tsquery,
'MinWords=5');
         ts_headline
-----------------------------
 <b>1</b> 2 3 4 5 6 7 8 9 10
(1 row)

-Sushant.