fix: fix NullPointerException

fix NullPointerException in ContractStatusRecorder.killContract when killing a nonexistent contract process

src/main/java/org/bdware/sc/ContractStatusRecorder.java

@@ -229,7 +229,9 @@ public class ContractStatusRecorder extends StatusRecorder<ContractMeta> {
             ContractManager.instance.invokeContractSuicide(client);
             ContractManager.cPort.updateDb(client.port, false);
         }
+        if (null != meta.contractExecutor) {
             meta.contractExecutor.close();
+        }
         id2ContractClient.remove(meta.id);
         runningProcess.remove(meta);
         return "success";