Bug in tsvector stats collection, fixed.

There was a small bug in the new compute_tsvector_stats function in CVS
HEAD, causing a crash with this surprisingly simple test case:

postgres=# CREATE TABLE tstest (ts tsvector);
CREATE TABLE
postgres=# INSERT INTO tstest values ('foobar');
INSERT 0 1
postgres=# ANALYZE tstest;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

The crash happens here:

/* Grab the minimal and maximal frequencies that will get stored */
minfreq = sort_table[num_mcelem - 1]->frequency;
maxfreq = sort_table[0]->frequency;

Because the loop above this point suppresses items that occur only once,
num_mcelem is 0.

The fix is trivial, but I thought I'd post this test case anyway to get
it archived. The patch I committed moves the code above within the
"if(num_mcelem > 0)" block below that. The possibility that num_mcelem
can be zero was clearly thought of, but the check was too late.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com