Fix: sessiond: double socket close on allocation failure

Fix: sessiond: double socket close on allocation failure

The application registration thread performs a double close() on
an application socket whenever it fails to allocate a ust_command.

Assign `-1` to `sock` after the initial close() to follow the
pattern of other close paths.

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