Summary:
In one path of BlockBasedTable::MultiGet(), Next() is directly called after calling Seek() against the index iterator. This might cause crash if an I/O error happens in Seek().
The bug is discovered in crash test.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9993

Test Plan: See existing CI tests pass.

Reviewed By: anand1976

Differential Revision: D36381758

fbshipit-source-id: a11e0aa48dcee168c2554c33b532646ffdb68877