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

Hi,

I found that fetch_more_data_begin() in postgres_fdw reports an error when PQsendQuery() returns the value less than 0
asfollows though PQsendQuery() can return only 1 or 0. I think this is  a bug. Attached is the patch that fixes this
bug.This needs to be back-ported to v14 where async execution was supported in postgres_fdw.
 

    if (PQsendQuery(fsstate->conn, sql) < 0)
        pgfdw_report_error(ERROR, NULL, fsstate->conn, false, fsstate->query);

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION