Fix memory leak when registration fails

registration_data_t is not freed if the server does not respond to the
connection message or the user decides to quit the (Wakaama) client
before the server respond.

This fixes #693.

core/liblwm2m.c

@@ -173,6 +173,9 @@ void prv_deleteTransactionList(lwm2m_context_t * context)
 
         transaction = context->transactionList;
         context->transactionList = context->transactionList->next;
+        if (transaction->callback) {
+            transaction->callback(context, transaction, NULL);
+        }
         transaction_free(transaction);
     }
 }