Fix bug in ExpandWhileOverlapping()

Summary: If ExpandWhileOverlapping() we don't clear inputs. That's a bug introduced by my recent patch https://reviews.facebook.net/D36687. However, we have no tests covering ExpandWhileOverlapping(). I created a task t6771252 to add ExpandWhileOverlapping() tests.

Test Plan: make check

Reviewers: sdong, rven, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37077

db/compaction_picker.cc

@@ -825,6 +825,8 @@ bool LevelCompactionPicker::PickCompactionBySize(VersionStorageInfo* vstorage,
     return false;
   }
 
+  inputs->clear();
+
   assert(level >= 0);
 
   // Pick the largest file in this level that is not already