reftable/block: use `size_t` to track restart point index

reftable/block: use `size_t` to track restart point index

The function `block_reader_restart_offset()` gets the offset of the
`i`th restart point. `i` is a signed integer though, which is certainly
not the correct type to track indices like this. Furthermore, both
callers end up passing a `size_t`.

Refactor the code to use a `size_t` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>