BUG #5189: Postgres Crash when trying to ts_stat a empty tsvector

The following bug has been logged online:

Bug reference:      5189
Logged by:          Brice Maron
Email address:      bmaron@gmail.com
PostgreSQL version: 8.4.1
Operating system:   Ubunut Linux 9.10 (2.6.31-14)
Description:        Postgres Crash when trying to ts_stat a empty tsvector
Details:

Description :

When  i try to get ts_stats about a empty tsvector,
postgresql crash....

Reproduce it :

SELECT * FROM ts_stat('SELECT to_tsvector('''')');