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

contrib/postgres_fdw/postgres_fdw.c

@@ -7082,7 +7082,7 @@ fetch_more_data_begin(AsyncRequest *areq)
 	snprintf(sql, sizeof(sql), "FETCH %d FROM c%u",
 			 fsstate->fetch_size, fsstate->cursor_number);
 
-	if (PQsendQuery(fsstate->conn, sql) < 0)
+	if (!PQsendQuery(fsstate->conn, sql))
 		pgfdw_report_error(ERROR, NULL, fsstate->conn, false, fsstate->query);
 
 	/* Remember that the request is in process */