DO not report level size as zero when there are no files in L0

Summary:
Instead of checking for number of files in L0. Check for number of files in the requested level.

Bug introduced in D4929 (diff trying to do too many things).

Test Plan: db_test.

Reviewers: dhruba, MarkCallaghan

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D9483

db/version_set.cc

@@ -1657,10 +1657,8 @@ void VersionSet::AddLiveFilesCurrentVersion(std::set<uint64_t>* live) {
 int64_t VersionSet::NumLevelBytes(int level) const {
   assert(level >= 0);
   assert(level < NumberLevels());
-  if(current_ && level < (int)current_->files_->size())
+  assert(current_);
-    return TotalFileSize(current_->files_[level]);
+  return TotalFileSize(current_->files_[level]);
-  else
-    return 0;
 }
 
 int64_t VersionSet::MaxNextLevelOverlappingBytes() {