Summary:
Fix seg fault in auto_readahead_size
```
db_stress:
internal_repo_rocksdb/repo/table/block_based/partitioned_index_iterator.h:70: virtual rocksdb::IndexValue rocksdb::PartitionedIndexIterator::value() const: Assertion `Valid()' failed.
```

During seek, after calculating readahead_size, db_stress can inject IOError resulting in failure to index_iter_->Seek and making index_iter_ invalid.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11761

Test Plan: Reproducible locally and passed with this fix

Reviewed By: anand1976

Differential Revision: D48696248

Pulled By: akankshamahajan15

fbshipit-source-id: 2be43bf56ad0fc2f95f9093c19c9a1b15a716091