Core/Unit: Always initialize a pointer before using it.

(less segfaults, more fun)

src/server/game/Entities/Unit/Unit.cpp

@@ -252,6 +252,7 @@ m_HostileRefManager(this)
 
     _focusSpell = NULL;
     _targetLocked = false;
+    _lastLiquid = NULL;
 }
 
 ////////////////////////////////////////////////////////////