pgsql: Fix missed cases in libpq's error handling.

Fix missed cases in libpq's error handling.

Commit 618c16707 invented an "error_result" flag in PGconn, which
intends to represent the state that we have an error condition and
need to build a PGRES_FATAL_ERROR PGresult from the message text in
conn->errorMessage, but have not yet done so.  (Postponing construction
of the error object simplifies dealing with out-of-memory conditions
and with concatenation of messages for multiple errors.)  For nearly all
purposes, this "virtual" PGresult object should act the same as if it
were already materialized.  But a couple of places in fe-protocol3.c
didn't get that memo, and were only testing conn->result as they used
to, without also checking conn->error_result.

In hopes of reducing the probability of similar mistakes in future,
I invented a pgHavePendingResult() macro that includes both tests.

Per report from Peter Eisentraut.

Discussion: https://postgr.es/m/b52277b9-fa66-b027-4a37-fb8989c73ff8@enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/914611ea738a3601717990faff0f5d71a0f14a3d

Modified Files
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src/interfaces/libpq/fe-exec.c      |  5 +++--
src/interfaces/libpq/fe-protocol3.c | 10 +++++-----
src/interfaces/libpq/libpq-int.h    |  8 ++++++++
3 files changed, 16 insertions(+), 7 deletions(-)