fix: fleeing entities must have a CharacterMovement component

[skaldarnar]

Problem: An entity with a death animation may already be stripped off its CharacterMovementComponent when receiving additional damage. This may cause an NPE when accessing the component without guarding check.

Solution: Require the CharacterMovementComponent in the event handler to ensure that it is present. Skip the event handler if the component is not present, as fleeing without character movement does make much sense anyways.

[skaldarnar]

@casals @jdrueckert I printed the full description (entity dump) of the entity right before the crash and noticed that the CharacterMovementComponent was just being removed from the entity. Upon closer inspection I noticed that it holds the DestroyAtAnimationEndComponent component, which is added by the same piece of code that already removes the character movement component. If I understand it correctly this is to avoid having the entity move while their death animation is played.

https://github.com/Terasology/WildAnimals/blob/develop/src/main/java/org/terasology/wildAnimals/Death/DeathSystem.java#L68-L91

This might just hint to another underlying issue, though. Please also check out Terasology/Health#98 which tries to fix one possible root cause that might confuse systems (unnecessarily).