ust-consumer: fix: metadata stream lock taken before destroy

The lock of a metadata stream is taken when calling the stream's
destroy function after the completion of a snapshot. This is invalid
and does not appear to protect anything.

I am guessin that this was meant to be an 'unlock' invoked in the
error path of this functions.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>