fix(presence_server_tester): avoid segfault

tester/presence_server_tester.c

@@ -428,6 +428,7 @@ static void test_presence_list_base(bool_t enable_compression) {
 	BC_ASSERT_EQUAL(laure->stat.number_of_NotifyPresenceReceived, 2, int, "%d");
 	BC_ASSERT_EQUAL(linphone_friend_list_get_expected_notification_version(linphone_core_get_default_friend_list(laure->lc)), 1, int, "%d");
 	lf = linphone_friend_list_find_friend_by_uri(linphone_core_get_default_friend_list(laure->lc), marie_identity);
+	if (!BC_ASSERT_PTR_NOT_NULL(lf)) goto end;
 	BC_ASSERT_EQUAL(linphone_friend_get_status(lf), LinphoneStatusBusy, int, "%d");
 	if (!BC_ASSERT_TRUE(linphone_friend_is_presence_received(lf))) goto end;
 	lf = linphone_friend_list_find_friend_by_uri(linphone_core_get_default_friend_list(laure->lc), pauline_identity);