Fix uninitlized CompactionJob::SubcompactionState::current_output_file_size

Summary: The new variable introduced in 2149059f910149197d1a0f79ac08cf19465ea2d may be unitialized. Valgrind is failing because of it.

Test Plan: Run valgrind tests

Reviewers: yiwu, andrewkr, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D63201

db/compaction_job.cc

@@ -145,6 +145,7 @@ struct CompactionJob::SubcompactionState {
     outputs = std::move(o.outputs);
     outfile = std::move(o.outfile);
     builder = std::move(o.builder);
+    current_output_file_size = std::move(o.current_output_file_size);
     total_bytes = std::move(o.total_bytes);
     num_input_records = std::move(o.num_input_records);
     num_output_records = std::move(o.num_output_records);