Loot: fix 'use after free'. Loot objects have ownership of their LootItem objects, and also keep track of any associated GroupLootRoll objects. A GroupLootRoll object in turn references the LootItem the roll is about, and its destructor may still use the LootItem. So when a Loot object is destructed, it can't delete its LootItem objects as long as any GroupLootRoll objects are still around that may reference/use them! Fixes cmangos/issues#3860.