pgsql: postgres_fdw: Fix bug in checking of return value of PQsendQuery

postgres_fdw: Fix bug in checking of return value of PQsendQuery().

When postgres_fdw begins an asynchronous data fetch, it submits FETCH query
by using PQsendQuery(). If PQsendQuery() fails and returns 0, postgres_fdw
should report an error. But, previously, postgres_fdw reported an error
only when the return value is less than 0, though PQsendQuery() never return
the values other than 0 and 1. Therefore postgres_fdw could not handle
the failure to send FETCH query in an asynchronous data fetch.

This commit fixes postgres_fdw so that it reports an error
when PQsendQuery() returns 0.

Back-patch to v14 where asynchronous execution was supported in postgres_fdw.

Author: Fujii Masao
Reviewed-by: Japin Li, Tom Lane
Discussion: https://postgr.es/m/b187a7cf-d4e3-5a32-4d01-8383677797f3@oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/44ccdce514c4b3b92c8aca744e9cf7be4580be0b

Modified Files
--------------
contrib/postgres_fdw/postgres_fdw.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)