Http interface: assertion failed in VLM when starting a stream

Reproduce:

• build with assertions
• open "manage stream"
• add a new stream with default parameter and random name
• click on the "play" button of the newly created stream

The following assertion is raised.

vlc: ../../src/input/vlm.c:106: player_on_state_changed: Assertion `psz_instance_name' failed.