Core/AI: Fix CreatureAI left uninitialized

Fix CreatureAI left uninitialized for 1 server tick for Creatures with DeathState:DEAD . This is a workaround required after c9cf2f059f8f33c3fcb58ad472871515e59df7c3 changes .

Valgrind log:
 Conditional jump or move depends on uninitialised value(s)
  at : npc_draenei_survivor::npc_draenei_survivorAI::MoveInLineOfSight(Unit*) (zone_azuremyst_isle.cpp:104)
  by : CreatureAI::MoveInLineOfSight_Safe(Unit*) (CreatureAI.cpp:123)
  by : CreatureUnitRelocationWorker(Creature*, Unit*) (GridNotifiers.cpp:134)
  by : Trinity::AIRelocationNotifier::Visit(GridRefManager<Creature>&) (GridNotifiers.cpp:252)
 Uninitialised value was created by a heap allocation
  at : operator new(unsigned long) (vg_replace_malloc.c:319)
  by : npc_draenei_survivor::GetAI(Creature*) const (zone_azuremyst_isle.cpp:66)
  by : ScriptMgr::GetCreatureAI(Creature*) (ScriptMgr.cpp:792)
  by : FactorySelector::selectAI(Creature*) (CreatureAISelector.cpp:41)
  by : Creature::AIM_Initialize(CreatureAI*) (Creature.cpp:716)
  by : Creature::AddToWorld() (Creature.cpp:190)