Avoid div-by-zero error in db_stress (facebook#9086)

Summary:
If a column family has 0 levels, then existing `TestCompactFiles(...)` may hit
divide-by-zero. To fix, return early if the cf is empty.

Pull Request resolved: facebook#9086

Test Plan: TBD

Reviewed By: ajkr

Differential Revision: D31986799

Pulled By: riversand963

fbshipit-source-id: 48f7dfb2b2b47cfc1315cb71ca80eb230d947f17

db_stress_tool/db_stress_test_base.cc

@@ -1812,6 +1812,10 @@ void StressTest::TestCompactFiles(ThreadState* thread,
   ROCKSDB_NAMESPACE::ColumnFamilyMetaData cf_meta_data;
   db_->GetColumnFamilyMetaData(column_family, &cf_meta_data);
 
+  if (cf_meta_data.levels.empty()) {
+    return;
+  }
+
   // Randomly compact up to three consecutive files from a level
   const int kMaxRetry = 3;
   for (int attempt = 0; attempt < kMaxRetry; ++attempt) {