Fix: remove the potential dead store variable in block_based_table_reader.cc (#6204)

Summary:
buf_offset does not need to get the value from req.len for othe final block. It can cause test fail for clan_analyze. Remove it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6204

Test Plan: pass make asan_check

Differential Revision: D19145335

Pulled By: zhichao-cao

fbshipit-source-id: 8f6e74565746381b5c5ef598b97d746517b36e5b

table/block_based/block_based_table_reader.cc

@@ -2374,7 +2374,6 @@ void BlockBasedTable::RetrieveMultipleBlocks(
       req.scratch = new char[req.len];
     } else {
       req.scratch = scratch + buf_offset;
-      buf_offset += req.len;
     }
     req.status = IOStatus::OK();
     read_reqs.emplace_back(req);