Fix: sessiond: registry_channel: initialize _rcu_head and _node

1490020 Uninitialized pointer field
The pointer field will point to an arbitrary memory location, any
attempt to write may cause corruption.

In lttng::​sessiond::​ust::​registry_channel::​registry_channel(unsigned int, std::​function<void (lttng::​sessiond::​ust::​registry_channel const &)>, std::​function<void (lttng::​sessiond::​ust::​registry_channel const &, lttng::​sessiond::​ust::​registry_event const &)>): A pointer field is not initialized in the constructor (CWE-457)

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie043af750941a02a65ba69e70fe2620349989398