Add bounds check in FilePickerMultiGet::PrepareNextLevel() (#5189)

Summary:
Add bounds check when looping through empty levels in FilePickerMultiGet
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5189

Differential Revision: D14925334

Pulled By: anand1976

fbshipit-source-id: 65d53247cf443153e28ce2b8b753fa51c6ae4566

db/version_set.cc

@@ -672,8 +672,11 @@ class FilePickerMultiGet {
           fp_ctx.search_right_bound = FileIndexer::kLevelMaxIndex;
         }
         // Skip all subsequent empty levels
-        while ((*level_files_brief_)[++curr_level_].num_files == 0) {
-        }
+        do {
+          ++curr_level_;
+        } while ((curr_level_ < num_levels_) &&
+                 (*level_files_brief_)[curr_level_].num_files == 0);
+        continue;
       }
 
       // Some files may overlap each other. We find