reftable/block: fix use of uninitialized memory when binsearch fails

reftable/block: fix use of uninitialized memory when binsearch fails

When doing the binary search through our restart offsets we may hit an
error in case `restart_needle_less()` fails to decode the record at the
given offset. While we correctly detect this case and error out, it will
cause us to call `reftable_record_release()` on the yet-uninitialized
record.

Fix this by initializing the record earlier.

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