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.

src/bin/pg_amcheck/pg_amcheck.c

@@ -958,6 +958,8 @@ should_processing_continue(PGresult *res)
 			/* This is expected but requires closer scrutiny */
 		case PGRES_FATAL_ERROR:
 			severity = PQresultErrorField(res, PG_DIAG_SEVERITY_NONLOCALIZED);
+			if (severity == NULL)
+				return false;	/* libpq failure, probably lost connection */
 			if (strcmp(severity, "FATAL") == 0)
 				return false;
 			if (strcmp(severity, "PANIC") == 0)