Fix: lock stream class after assigning stream id

Fixes a bug that was introduced by 2f100782 which made it possible
to set custom stream class IDs.

The stream class is frozen when a stream of its type is instanciated.
However, the trace or writer must still assign a unique ID to the stream
class if none were set prior. This modification moves the stream class
locking (freeze) after the ID assignment check.

Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>