pgsql: dblink: Fix variable confusion introduced in e4602483e95

dblink: Fix variable confusion introduced in e4602483e95

Thanks to Robins to find the issue and Nathan for promptly writing a test case
to prevent future problems like this.

Reported-by: Nathan Bossart <nathandbossart@gmail.com>
Reported-by: Robins Tharakan <tharakan@gmail.com>
Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: /messages/by-id/20230130193008.GA2740781@nathanxps13

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/fbed4bc59e362e13fb0d0e0b08c995c18c7e4d5a

Modified Files
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contrib/dblink/dblink.c | 2 +-
contrib/dblink/expected/dblink.out | 19 +++++++++++++++++++
contrib/dblink/sql/dblink.sql | 5 +++++
3 files changed, 25 insertions(+), 1 deletion(-)