Fixed boss evade and doors closed by cr4st.

Should close #6377 and #15239

(cherry picked from commit e430f3f61ae45ca0e8e251f96a1605bdb9299eb1)

src/server/scripts/EasternKingdoms/Stratholme/boss_baron_rivendare.cpp

@@ -106,7 +106,8 @@ public:
         void AttackStart(Unit* who) override
         {
             //can't use entercombat(), boss' dmg aura sets near players in combat, before entering the room's door
-            instance->SetData(TYPE_BARON, IN_PROGRESS);
+            if (instance && instance->GetData(TYPE_BARON) == NOT_STARTED)
+                instance->SetData(TYPE_BARON, IN_PROGRESS);
             ScriptedAI::AttackStart(who);
         }