reftable/blocksource: adjust type of the block length

The block length is used to track the number of bytes available in a
specific block. As such, it is never set to a negative value, but is
still represented by a signed integer.

Adjust the type of the variable to be `size_t`.

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

reftable/reftable-blocksource.h

@@ -22,7 +22,7 @@ struct reftable_block_source {
  * so it can return itself into the pool. */
 struct reftable_block {
 	uint8_t *data;
-	int len;
+	size_t len;
 	struct reftable_block_source source;
 };