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

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>