fix delete range bug

Summary:
Fix this [issue](https://github.com/facebook/rocksdb/issues/2989).
ajkr PTAL

Close #2989
Closes https://github.com/facebook/rocksdb/pull/3017

Differential Revision: D6078541

Pulled By: yiwu-arbug

fbshipit-source-id: ef3db87b37b9156f83ca468aa39dea1f6dbde49d

db/compaction_job.cc

@@ -1041,6 +1041,7 @@ Status CompactionJob::FinishCompactionOutputFile(
   auto meta = &sub_compact->current_output()->meta;
   if (s.ok()) {
     Slice lower_bound_guard, upper_bound_guard;
+    std::string smallest_user_key;
     const Slice *lower_bound, *upper_bound;
     if (sub_compact->outputs.size() == 1) {
       // For the first output table, include range tombstones before the min key
@@ -1050,7 +1051,8 @@ Status CompactionJob::FinishCompactionOutputFile(
       // For subsequent output tables, only include range tombstones from min
       // key onwards since the previous file was extended to contain range
       // tombstones falling before min key.
-      lower_bound_guard = meta->smallest.user_key();
+      smallest_user_key = meta->smallest.user_key().ToString(false /*hex*/);
+      lower_bound_guard = Slice(smallest_user_key);
       lower_bound = &lower_bound_guard;
     } else {
       lower_bound = nullptr;