unexpected behaviour in shared media case

There is a problem in the shared media.

The problem occurs in the following scenario:

1. client 1 and client 2 do DESCRIBE (cached media will be used for client 2)
2. both clients do SETUP
3. client 1 does PLAY followed by TEARDOWN => media is finalized as there are no active transports after this stage)
4. client 2 does PLAY => crash (media is gone)

Running test_shared_tcp() in a loop, will sooner or later expose this problem.