Core/QuestPOI: Mitigate possible DoS with CMSG_QUEST_POI_QUERY

Avoid sending POIs for same quest if the client somehow sent duplicates quest id in same CMSG_QUEST_POI_QUERY packet. This also reduce the effects of possible DoS and increases the difficulty to cause it.
Fix a typo which caused no quest POIs to be sent at all if the client queried data for 25 quests.