[Core] Fix Error Handling Super Emoticon

[kengwang]

Motivation:

Lagrange will throw an error when trying to parse MessageElem in MentionEntity, says that:

Lagrange.Core.Exceptions.LagrangeException: An error occurred while processing the incoming event
 ---> System.Collections.Generic.KeyNotFoundException: Field 4 not found in ProtoObject.

Reproduce:

Send a super emoticon in chat.

obj only has one element that contains the fallback string.

Fixes

Check if it has the corresponding index.

[Linwenxuan04]

其实通过有没有4这个Field来判断不太靠谱 具体的逻辑因为NTQQ现在虚的太厉害了还没有逆出来 但是里面至少有一个字段应该是代表的type

或者 我们应该使用Attr6buf来判断这个是否为一个Mention消息