Summary:
A fix similar to #4410 but on the write path. On IO error on `SelectBlobFile()` we didn't return error code properly, but simply a nullptr of `BlobFile`. The `AppendBlob()` method didn't have null check for the pointer and caused crash. The fix make sure we properly return error code in this case.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4580
Differential Revision: D10513849
Pulled By: yiwu-arbug
fbshipit-source-id: 80bca920d1d7a3541149de981015ad83e0aa14b5