pg11+: pg_ls_*dir LIMIT 1: temporary files .. not closed atend-of-transaction

While working on a patch, I noticed this pre-existing behavior, which seems to
be new since v11, maybe due to changes to SRF.

|postgres=# SELECT pg_ls_dir('.') LIMIT 1;
|WARNING:  1 temporary files and directories not closed at end-of-transaction
|pg_ls_dir | pg_dynshmem

|postgres=# SELECT pg_ls_waldir() LIMIT 1;
|WARNING:  1 temporary files and directories not closed at end-of-transaction
|-[ RECORD 1 ]+-------------------------------------------------------------
|pg_ls_waldir | (00000001000031920000007B,16777216,"2020-03-08 03:50:34-07")


Note, that doesn't happen with "SELECT * FROM".

I'm not sure what the solution is to that, but my patch was going to make it
worse rather than better for pg_ls_tmpdir.

-- 
Justin