Fix: session: end goto label used for error path

Reported by Coverity:

    CID 1475801: Unused value (UNUSED_VALUE)
    Assigning value LTTNG_ERR_FATAL to ret_code here, but that stored value is overwritten before it can be used

    CID 1475802: Unused value (UNUSED_VALUE)
    Assigning value LTTNG_ERR_NOMEM to ret_code here, but that stored value is overwritten before it can be used.

    CID 1475814: Unused value (UNUSED_VALUE)
    Assigning value LTTNG_ERR_UST_CHAN_NOT_FOUND to ret_code here, but that stored value is overwritten before it can be used.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I79fb5d65ea0f4f3dfbbef2a42ec3dcf0542043c5