BUG #2574: C function: arg TEXT data corrupt

The following bug has been logged online:

Bug reference: 2574
Logged by: Michael Enke
Email address: michael(dot)enke(at)wincor-nixdorf(dot)com
PostgreSQL version: 8.1.4
Operating system: Open Suse 10.1 and CentOS 4.3
Description: C function: arg TEXT data corrupt
Details:

I created a C function:
extern Datum test_arg(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(test_arg);
Datum test_arg(PG_FUNCTION_ARGS) {
elog(INFO, "arg: %s", VARDATA(PG_GETARG_TEXT_P(0)));
PG_RETURN_INT16(0);
}
and used the
CREATE FUNCTION test_arg(TEXT) RETURNS INT4 AS 'path_to_lib.so' LANGUAGE
'C';

If I call this function "the first time"
after connecting with psql, the info output shows
corrupted data, the second and further call is ok:
me(at)noteme:~/uwx9/SVN/tpl/trunk/db/postgresql/src> psql -U tplinux
Welcome to psql 8.1.3, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit

tplinux=> select test_arg('1');
INFO: arg: 1s(at)3
test_arg
----------
0
(1 row)

tplinux=> select test_arg('1');
INFO: arg: 1
test_arg
----------
0
(1 row)

The same problem is with input length of 2, 3, 4, 8 char (and may be more).
With input of 4 char I never get correct value.

No problem for first call with 5, 6 , 7 char,
but with 5 char problem if before called with 6 char.