mediaplayer: fix vmem cleanup callback crash

A bug was introduced in https://github.com/videolan/libvlcsharp/pull/87 which would crash the CLR
when the cleanup callback was set. The issue was that the vmem plugin does not have a Stop concept,
only closed, in which it frees up a struct containing a pointer which is used later by LVS.

So for the cleanup callback only, revert to the previous code.

Closes videolan/LibVLCSharp#339