[mr3063] Optimize getVictim() usage (speed up things)

We have in Unit:
Unit* getVictim() const { return IsInWorld() ? GetMap()->GetUnit(m_attackingGuid) : NULL; }

In code many calls like:
if (getVictim())
    Do(getVictim());

So, everytime we search in objectsStore container (twice and more) the same unit, what takes time.
For speed need to find a one time only.