Fix: ctf: set `ret` on error paths

Fix: ctf: set `ret` on error paths

The error paths touched by this patch currently don't set the `ret`
variable, resulting in the error not being communicated to the caller.
Fix that.

Change-Id: If044e4ff28a98bda408610c1c2f85d7fe13ea1c1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/5691
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>