reftable/basics: fix OOB read on binary search of empty range

reftable/basics: fix OOB read on binary search of empty range

`binsearch()` performs a binary search over a range of `sz` elements by
repeatedly calling the comparison function with indices into that range.
When the range is empty though, there is no valid index to call the
comparison function with. We still end up executing the comparison
function though with an index of 0, which of course will cause an
out-of-bounds read.

Return early when the range is empty.

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