lttng enable-channel memsets lttng_channel to -1

The lttng enable-channel command uses a global
struct lttng_channel which it initializes to contain all "-1".

This is a hack used to set tag all attributes as "unset" which
has the unfortunate consequence of setting the extended pointer
to a non-sensical value. This workaround explicitly sets the
extended pointer to NULL, but this command should be refactored so
it is no longer necessary.

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