reftable/reader: be more careful about errors in indexed seeks

When doing an indexed seek we first need to do a linear seek in order to find the index block for our wanted key. We do not check the returned error of the linear seek though. This is likely not an issue because the next call to `table_iter_next()` would return error, too. But it very much is a code smell when an error variable is being assigned to without actually checking it. Safeguard the code by checking for errors. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>