Fix UniversalCompaction trivial move bug (#6067)

Summary:
`curr.level` is `c->inputs_` index, not real level.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6067

Differential Revision: D18935726

fbshipit-source-id: 4354e6e9cd900ca56c96e9d770f0ab6634e45daf

db/compaction/compaction_picker_universal.cc

@@ -247,7 +247,8 @@ bool UniversalCompactionBuilder::IsInputFilesNonOverlapping(Compaction* c) {
 
     next.f = nullptr;
 
-    if (curr.level != 0 && curr.index < c->num_input_files(curr.level) - 1) {
+    if (c->level(curr.level) != 0 &&
+        curr.index < c->num_input_files(curr.level) - 1) {
       next.f = c->input(curr.level, curr.index + 1);
       next.level = curr.level;
       next.index = curr.index + 1;