pgsql: libpq: Reset singlerow flag correctly in pipeline mode

libpq: Reset singlerow flag correctly in pipeline mode

When a query whose results were requested in single-row mode is the last
in the queue by the time those results are being read, the single-row
flag was not being reset, because we were returning early from
pqPipelineProcessQueue.  Move that stanza up so that the flag is always
reset at the end of sending that query's results.

Add a test for the situation.

Backpatch to 14.

Author: Denis Laxalde <denis.laxalde@dalibo.com>
Discussion: https://postgr.es/m/01af18c5-dacc-a8c8-07ee-aecc7650c3e8@dalibo.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/db1b931a4ee885ebd27bb5b335c1e461ccc34bbe

Modified Files
--------------
src/interfaces/libpq/fe-exec.c                     | 12 +++---
src/test/modules/libpq_pipeline/libpq_pipeline.c   | 45 +++++++++++++++++++++-
.../modules/libpq_pipeline/traces/singlerow.trace  | 20 ++++++++++
3 files changed, 70 insertions(+), 7 deletions(-)