Repostore crash debug (#521)

* repostore listBlocks reads too much data

* Add assert to make sure data is not queried

codex/stores/repostore.nim

@@ -272,7 +272,8 @@ method listBlocks*(
     of BlockType.Block: CodexBlocksKey
     of BlockType.Both: CodexRepoKey
 
-  without queryIter =? (await self.repoDs.query(Query.init(key))), err:
+  let query = Query.init(key, value=false)
+  without queryIter =? (await self.repoDs.query(query)), err:
     trace "Error querying cids in repo", blockType, err = err.msg
     return failure(err)
 
@@ -281,6 +282,7 @@ method listBlocks*(
     iter.finished = queryIter.finished
     if not queryIter.finished:
       if pair =? (await queryIter.next()) and cid =? pair.key:
+        doAssert pair.data.len == 0
         trace "Retrieved record from repo", cid
         return Cid.init(cid.value).option