Core/PetAI: Fix memory leak

Fix memory leak added in e7f0808d467210c2e0ca455cf810e52700518849

Valgrind log:
395,712 (390,216 direct, 5,496 indirect) bytes in 229 blocks are definitely lost in loss record 65 of 67
 at operator new(unsigned long) (vg_replace_malloc.c:320)
 by PetAI::UpdateAI(unsigned int) (PetAI.cpp:167)
 by Creature::Update(unsigned int) (Creature.cpp:541)
 by TempSummon::Update(unsigned int) (TemporarySummon.cpp:47)