Fix build error: variable 'base_level' may be uninitialized (#11435)

Summary:
Fix build error: variable 'base_level' may be uninitialized
```
 db_impl_compaction_flush.cc:1195:21: error: variable 'base_level' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
            level = base_level;
```
                    ^~~~~~~~~~

Pull Request resolved: #11435

Test Plan: CircleCI jobs

Reviewed By: cbi42

Differential Revision: D45708176

Pulled By: akankshamahajan15

fbshipit-source-id: 851b1205b22b63d728495e5735fa91b0ad8e012b

db/db_impl/db_impl_compaction_flush.cc

@@ -1170,7 +1170,7 @@ Status DBImpl::CompactRangeInternal(const CompactRangeOptions& options,
         // at L1 (or LBase), if applicable.
         int level = first_overlapped_level;
         final_output_level = level;
-        int output_level, base_level;
+        int output_level = 0, base_level = 0;
         while (level < max_overlapped_level || level == 0) {
           output_level = level + 1;
           if (cfd->ioptions()->level_compaction_dynamic_level_bytes &&