Summary:
Fix seg fault in auto_readahead_size with async_io when readahead_size = 0. If readahead_size is trimmed and is 0, it's not eligible for further prefetching and should return.

Error occured when the first buffer already contains data and it goes for prefetching in second buffer leading to assertion failure -

`assert(roundup_len1 >= alignment);
`
because roundup_len1 = length + readahead_size.
length is 0 and readahead_size is also 0.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11769

Test Plan: Reproducible with db_stress with async_io enabled.

Reviewed By: anand1976

Differential Revision: D48743031

Pulled By: akankshamahajan15

fbshipit-source-id: 0e08c41f862f6287ca223fbfaf6cd42fc97b3c87