Scripts/Shadowmoon Valley: Fix static analysis issues added in cd48ee01c0

Fix static analysis issues added in cd48ee01c0 about uninitialized values and pointers possibly point to free'd memory being dereferenced.
In memory of all the developers who got banned from IRC in the process of fixing these issues.

src/server/scripts/Outland/zone_shadowmoon_valley.cpp

@@ -71,7 +71,10 @@ public:
 
     struct npc_invis_infernal_casterAI : public ScriptedAI
     {
-        npc_invis_infernal_casterAI(Creature* creature) : ScriptedAI(creature) { }
+        npc_invis_infernal_casterAI(Creature* creature) : ScriptedAI(creature)
+        {
+            ground = 0.f;
+        }
 
         void Reset() override
         {
@@ -148,12 +151,12 @@ public:
         void IsSummonedBy(Unit* summoner) override
         {
             if (summoner->ToCreature())
-                caster = summoner->ToCreature();
+                casterGUID = summoner->ToCreature()->GetGUID();;
         }
 
         void JustDied(Unit* /*killer*/) override
         {
-            if (caster)
+            if (Creature* caster = ObjectAccessor::GetCreature(*me, casterGUID))
                 caster->AI()->SetData(TYPE_INFERNAL, DATA_DIED);
         }
 
@@ -175,7 +178,7 @@ public:
         }
 
     private:
-        Creature* caster;
+        ObjectGuid casterGUID;
     };
 
     CreatureAI* GetAI(Creature* creature) const override