Fix: sink.ctf.fs: dereference before NULL check

Fix: sink.ctf.fs: dereference before NULL check

  CID 1404550 (#1 of 1): Dereference before null check (REVERSE_INULL)
  check_after_deref: Null-checking tsdl suggests that it may be null,
  but it has already been dereferenced on all paths leading to the
  check.

Reported-by: Coverity - Dereference before null check (REVERSE_INULL)
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I7566cbd74529e93587079d163bb1691e420f723f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1974
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>