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>

Author: pks-t

reftable/block.c

@@ -517,6 +517,10 @@ int block_iter_seek_key(struct block_iter *it, struct reftable_buf *want)
 	int err = 0;
 	size_t i;
 
+	err = reftable_record_init(&rec, reftable_block_type(it->block));
+	if (err < 0)
+		goto done;
+
 	/*
 	 * Perform a binary search over the block's restart points, which
 	 * avoids doing a linear scan over the whole block. Like this, we
@@ -558,10 +562,6 @@ int block_iter_seek_key(struct block_iter *it, struct reftable_buf *want)
 	else
 		it->next_off = it->block->header_off + 4;
 
-	err = reftable_record_init(&rec, reftable_block_type(it->block));
-	if (err < 0)
-		goto done;
-
 	/*
 	 * We're looking for the last entry less than the wanted key so that
 	 * the next call to `block_reader_next()` would yield the wanted