Fix an incorrect MultiGet assertion (#10695)

Summary:
The assertion in ```FilePickerMultiGet::ReplaceRange()``` was incorrect. The function should only be called to replace the range after finishing the search in the current level, which is indicated by ```hit_file_ == nullptr``` i.e no more overlapping files in this level.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10695

Reviewed By: gitbw95

Differential Revision: D39583217

Pulled By: anand1976

fbshipit-source-id: d4cedfb2b62fb9f3a083e9848a403ae6342f0519

db/version_set.cc

@@ -511,7 +511,7 @@ class FilePickerMultiGet {
   MultiGetRange& GetRange() { return range_; }
 
   void ReplaceRange(const MultiGetRange& other) {
-    assert(curr_level_ == 0 || !RemainingOverlapInLevel());
+    assert(hit_file_ == nullptr);
     range_ = other;
     current_level_range_ = other;
   }