Fix pg_dump/pg_restore's ExecuteSqlCommand() to behave suitably if PQexec

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.