empty array can crash backend using int_array_enum from contrib.

Using the int_array_enum function from contrib/intagg I can crash the 8.0.2 backend when I pass it an empty array.

fli=# select int_array_enum('{}'::int[]);
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>

fli=# select * from version();
version
-------------------------------------------------------------------------------------
PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.3.3 (SuSE Linux)
(1 row)