[*] Add sign check

dap_chain_cs_dag_pos.c

@@ -232,6 +232,10 @@ static int s_callback_event_verify(dap_chain_cs_dag_t * a_dag, dap_chain_cs_dag_
             }
 
             dap_enc_key_t * l_key = dap_sign_to_enc_key( l_sign);
+            if ( l_key == NULL){
+                log_it(L_WARNING, "Event's sign has no any key");
+                return -5;
+            }
 
             dap_chain_addr_fill (&l_addr,l_key,&a_dag->chain->net_id );
             dap_enc_key_delete (l_key); // TODO cache all this operations to prevent useless memory copy ops