Fix: use of uninitialized channel attributes in client

The channel attributes used to retrieve the defaults to use
are not zero-initialized which is a problem now that the extended
pointer is used to determine whether or not the structure was
allocated on stack by the user (for backward compatibility) or
dynamically using the new lttng_channel_create() interface.

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