Some pointer safety in threat manager

Querying nullptr shouldn't even attempt to return anything meaningful.
Passively helps with crashes when threat manager is used carelessly, getVitctim() case in particular.

(based on cmangos/mangos-wotlk@cfb37f659)

Signed-off-by: Xfurry <xfurry.cmangos@outlook.com>

src/game/ThreatManager.cpp

@@ -224,6 +224,8 @@ void ThreatContainer::clearReferences()
 // Return the HostileReference of nullptr, if not found
 HostileReference* ThreatContainer::getReferenceByTarget(Unit* pVictim)
 {
+    if (!pVictim)
+        return nullptr;
     HostileReference* result = nullptr;
     ObjectGuid guid = pVictim->GetObjectGuid();
     for (ThreatList::const_iterator i = iThreatList.begin(); i != iThreatList.end(); ++i)