Core/Movement: interrupt a creature's current cast when fleeing for assistance, and (if spell is not castable while moving) when running random/fleeing movement generators. (#17271)

Also make sure the creature is alive before updating.

Fixes #12823.

src/server/game/Movement/MotionMaster.cpp

@@ -531,6 +531,7 @@ void MotionMaster::MoveSeekAssistance(float x, float y, float z)
         TC_LOG_DEBUG("misc", "Creature (Entry: %u GUID: %u) seek assistance (X: %f Y: %f Z: %f)",
             _owner->GetEntry(), _owner->GetGUID().GetCounter(), x, y, z);
         _owner->AttackStop();
+        _owner->CastStop();
         _owner->ToCreature()->SetReactState(REACT_PASSIVE);
         Mutate(new AssistanceMovementGenerator(x, y, z), MOTION_SLOT_ACTIVE);
     }