fix deprecated copy constructor warning in StreamBuffer::ContainedLink

I saw some deprecated copy constructor warnings, the code looks to me like it hadn't undergone the typical modernizations we have elsewhere in the code (default values for data members instead of spelled out default constructor, hand-implemented default copy constructor, no copy assignment …). For reference, I was looking at https://i.stack.imgur.com/8Yzpz.png and clang-tidy here (or here). The style ( = for initialization vs. {} mimics what I saw further down the file.