reftable/block: fix error handling when searching restart points

reftable/block: fix error handling when searching restart points

When doing the binary search over restart points in a block we need to
decode the record keys. This decoding step can result in an error when
the block is corrupted, which we indicate to the caller of the binary
search by setting `args.error = 1`. But the only caller that exists
mishandles this because it in fact performs the error check before
calling `binsearch()`.

Fix this bug by checking for errors at the right point in time.
Furthermore, refactor `binsearch()` so that it aborts the search in case
the callback function returns a negative value so that we don't
needlessly continue to search the block.

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