mission_t* mission = NULL;

[...]

while (list) {
	mission = (mission_t *)list->data;
	/* check whether current selected gametype is a valid one */
	if (!Q_strcmp(SEARCH_MISSION_ID_STRING, mission->id)) {
		break;
	}
	list = list->next;
}

if (mission != NULL)
/* found */
else
/* not found */