Fix DBSSTest::AddExternalSstFileSkipSnapshot valgrind fail

Summary: Fix the test by releasing the last snapshot

Test Plan: run the test under valgrind

Reviewers: andrewkr, yiwu, lightmark, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62091

db/db_sst_test.cc

@@ -1345,6 +1345,8 @@ TEST_F(DBSSTTest, AddExternalSstFileSkipSnapshot) {
   ASSERT_OK(db_->AddFile(std::vector<ExternalSstFileInfo>(1, file3_info), false, true));
   ASSERT_EQ(Get(Key(300)), Key(300) + ("_val"));
   ASSERT_EQ(Get(Key(300), s2), Key(300) + ("_val"));
+
+  db_->ReleaseSnapshot(s2);
 }
 
 TEST_F(DBSSTTest, AddExternalSstFileMultiThreaded) {