Fix dangling checkpoint pointer in db_stress (#4042)

Summary:
Fix db_stress failed to delete checkpoint pointer. It's caught by asan_crash test.
Closes https://github.com/facebook/rocksdb/pull/4042

Differential Revision: D8592604

Pulled By: yiwu-arbug

fbshipit-source-id: 7b2d67d5e3dfb05f71c33fcf320482303e97d3ef

tools/db_stress.cc

@@ -1794,6 +1794,7 @@ class StressTest {
           s = FLAGS_env->GetChildren(checkpoint_dir, &files);
         }
         DestroyDB(checkpoint_dir, Options());
+        delete checkpoint;
         if (!s.ok()) {
           printf("A checkpoint operation failed with: %s\n",
                  s.ToString().c_str());