Don't fail on libpq-generated error reports in pg_amcheck.
An error PGresult generated by libpq itself, such as a report of
connection loss, won't have broken-down error fields.
should_processing_continue() blithely assumed that
PG_DIAG_SEVERITY_NONLOCALIZED would always be present, and would
dump core if it wasn't.
Per grepping to see if
6d157e7cb's mistake was repeated elsewhere.