Fix clang-analyze by adding assertion (#9682)

Summary:
Clang-analyze complains about potential nullptr dereference.
Fix by adding an assertion to make clang happy.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9682

Test Plan: USE_CLANG=1 make -j20 analyze_incremental

Reviewed By: ltamasi

Differential Revision: D34755210

Pulled By: riversand963

fbshipit-source-id: 948e1899846ee1aa05a1b500a11ff43b0b412e0a

utilities/transactions/write_committed_transaction_ts_test.cc

@@ -263,6 +263,7 @@ TEST_P(WriteCommittedTxnWithTsTest, TransactionDbLevelApi) {
   PutFixed64(&ts_str, 100);
   Slice value = value_str;
 
+  assert(db);
   ASSERT_TRUE(
       db->Put(WriteOptions(), handles_[1], "foo", "bar").IsNotSupported());
   ASSERT_TRUE(db->Delete(WriteOptions(), handles_[1], "foo").IsNotSupported());