pgsql: Fix pg_dump/pg_restore's ExecuteSqlCommand() to behave suitably

Log Message:
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Fix pg_dump/pg_restore's ExecuteSqlCommand() to behave suitably if PQexec
returns NULL instead of a PGresult.  The former coding would fail, which
is OK, but it neglected to give you the PQerrorMessage that might tell
you why.  In the oldest branches, there was another problem: it'd sometimes
report PQerrorMessage from the wrong connection.

Tags:
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REL8_2_STABLE

Modified Files:
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    pgsql/src/bin/pg_dump:
        pg_backup_db.c (r1.75 -> r1.75.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_db.c?r1=1.75&r2=1.75.2.1)