Core/SmartAI: store event invoker when available, and use it for Invoker target

note: dont forget to call script_reset when script is over!

--HG--
branch : trunk

src/server/game/AI/SmartScripts/SmartScript.h

@@ -214,6 +214,7 @@ class SmartScript
         uint64 mTextGUID;
         Creature* talker;
         bool mUseTextTimer;
+        Unit* mLastInvoker;
 
         SMARTAI_TEMPLATE mTemplate;
         void InstallEvents();