Summary:
Fix a bug in the calculation of the input buffer address/offset in log_reader.cc. The bug is when consecutive fragments of a compressed record are located at the same offset in the log reader buffer, the second fragment input buffer is treated as a leftover from the previous input buffer. As a result, the offset in the `ZSTD_inBuffer` is not reset.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11198

Test Plan: Add a unit test in log_test.cc that fails without the fix and passes with it.

Reviewed By: ajkr, cbi42

Differential Revision: D43102692

Pulled By: anand1976

fbshipit-source-id: aa2648f4802c33991b76a3233c5a58d4cc9e77fd