Fix use-after-free in creature conditions

[diath]

The Creature class destructor loops over the conditions member variable and calls condition->endCondition then deletes the condition object, however, ConditionInvisible::endCondition calls Creature::isInvisible, which subsequently loops over the conditions (which are not erased when being deleted in the destructor), calling condition->getType on an already deleted memory, resulting in UAF. This fixes the issue by first ending all conditions, then deleting them separately (as erasing the elements one-by-one in the Creature destructor does not make much sense).

[ranisalt]

Good catch. It is probably better to use smart pointers here, but this is a good stop-gap.