Store pendingInstanceId instead of pointer of instance save, because pointer may become invalid. Fix #2987.

src/server/game/Entities/Player/Player.cpp

@@ -1754,7 +1754,7 @@ void Player::Update(uint32 p_time)
         if (_pendingBindTimer <= p_time)
         {
             // Player left the instance
-            if (_pendingBind->GetInstanceId() == GetInstanceId())
+            if (_pendingBindId == GetInstanceId())
                 BindToInstance();
             SetPendingBind(NULL, 0);
         }
@@ -18021,10 +18021,14 @@ InstancePlayerBind* Player::BindToInstance(InstanceSave* save, bool permanent, b
 
 void Player::BindToInstance()
 {
+    InstanceSave* mapSave = sInstanceSaveMgr->GetInstanceSave(_pendingBindId);
+    if (!mapSave) //it seems sometimes mapSave is NULL, but I did not check why
+        return;
+
     WorldPacket data(SMSG_INSTANCE_SAVE_CREATED, 4);
     data << uint32(0);
     GetSession()->SendPacket(&data);
-    BindToInstance(_pendingBind, true);
+    BindToInstance(mapSave, true);
 }
 
 void Player::SendRaidInfo()