Fix: consumerd: crash occurs when taking snapshot of ust channel

Commit 8e1ef46e8 added an acquisition of the metadata_stream's lock
during consumer_metadata_cache_flushed() as stream attributes are
used. However, when this function is called, the metadata channel's
stream can already be NULL, as indicated by the function's comments.

Check if the stream is NULL before attempting to acquire its lock.

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