res_pjsip_pubsub: On notify fail deleted sub_tree is then referenced

This change makes the send_notify of the sub_tree
not happen when the sub_tree has been deleted due
to the notify call failing, which avoids a crash.

ASTERISK-24970 #close

Change-Id: I1f20ffc08b192f59c457293b218025a693992cbf

res/res_pjsip_pubsub.c

@@ -2601,12 +2601,13 @@ static pj_bool_t pubsub_on_rx_subscribe_request(pjsip_rx_data *rdata)
 		sip_subscription_accept(sub_tree, rdata, resp);
 		if (generate_initial_notify(sub_tree->root)) {
 			pjsip_evsub_terminate(sub_tree->evsub, PJ_TRUE);
-		}
+		} else {
 			send_notify(sub_tree, 1);
 			ast_test_suite_event_notify("SUBSCRIPTION_ESTABLISHED",
 					"Resource: %s",
 					sub_tree->root->resource);
 		}
+	}
 
 	resource_tree_destroy(&tree);
 	return PJ_TRUE;