coalesce in execute crashes backend

Vinod Kurup (vkurup@massmed.org) reports a bug with a severity of 3
The lower the number the more severe it is.

Short Description
coalesce in execute crashes backend

Long Description
When I use a coalesce statement to calculate an expression that is part of a dynamic query, it crashes the backend.
Interestingly,it works if I use constants as arguments to coalesce.  

Sample Code
select version();

create function vk_test()
returns integer as '
declare
    v_id        integer;
begin
    v_id := 2;

    raise notice ''try a constant.'';
    execute ''select '' || coalesce(2,0) || '';'';

    raise notice ''that worked'';

    raise notice ''now, try a var with the same value.'';
    execute ''select '' || coalesce(v_id,0) || '';'';

    raise notice ''we never get here'';
    return 0;
end;' language 'plpgsql';


select vk_test();

                           version
-------------------------------------------------------------
 PostgreSQL 7.1 on i586-pc-linux-gnu, compiled by GCC 2.95.3
(1 row)

CREATE
psql:junk.sql:24: NOTICE:  try a constant.
psql:junk.sql:24: NOTICE:  that worked
psql:junk.sql:24: NOTICE:  now, try a var with the same value.
psql:junk.sql:24: pqReadData() -- backend closed the channel unexpectedly.
    This probably means the backend terminated abnormally
    before or while processing the request.
psql:junk.sql:24: connection to server was lost

server.log:
NOTICE:  try a constant.
NOTICE:  that worked
NOTICE:  now, try a var with the same value.
Server process (pid 7516) exited with status 11 at Tue Jun 12 14:32:13 2001
Terminating any active server processes...
NOTICE:  Message from PostgreSQL backend:
    The Postmaster has informed me that some other backend    died abnormally and possibly corrupted shared memory.
    I have rolled back the current transaction and am    going to terminate your database system connection and exit.
    Please reconnect to the database system and repeat your query.


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