add assert to silence clang warning (#4871)

Summary:
currently clang analyze fails with the following warning:
> utilities/transactions/write_prepared_transaction_test.cc:1451:5: warning: Forming reference to null pointer
    ASSERT_GT(wp_db->max_evicted_seq_, 0);  // max after recovery
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4871

Differential Revision: D13638053

Pulled By: miasantreble

fbshipit-source-id: b192b0c13c411c58defc9e280b34cdfcab3fa8e3

utilities/transactions/write_prepared_transaction_test.cc

@@ -1448,6 +1448,7 @@ TEST_P(WritePreparedTransactionTest, IsInSnapshotEmptyMapTest) {
     db_impl->FlushWAL(true);
     ReOpenNoDelete();
     WritePreparedTxnDB* wp_db = dynamic_cast<WritePreparedTxnDB*>(db);
+    assert(wp_db != nullptr);
     ASSERT_GT(wp_db->max_evicted_seq_, 0);  // max after recovery
     // Take a snapshot right after recovery
     const Snapshot* snap = db->GetSnapshot();