Fix libpq example programs

Peter Eisentraut <peter_e@gmx.net>

Commit: 7bdea6263543c0dabf4d374b3c8d2c3a1d91649e
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-07-01T12:06:40Z
Releases: 12.0
Fix libpq example programs

When these programs call pg_catalog.set_config, they need to check for
PGRES_TUPLES_OK instead of PGRES_COMMAND_OK.  Fix for
5770172cb0c9df9e6ce27c507b449557e5b45124.

Reported-by: Ideriha, Takeshi <ideriha.takeshi@jp.fujitsu.com>

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