src.ctf.fs: remove leftover use of pointer arithmetics

src.ctf.fs: remove leftover use of pointer arithmetics

This change should have been include in the commit:
  commit de38c26a14b60cb3b6d31cc124c187e2c1816bf5
  Author: Francis Deslauriers <francis.deslauriers@efficios.com>
  Date:   Fri Jul 5 11:19:32 2019 -0400

      Fix: src.ctf.fs: pointer arithmetics on non-adjacent memory

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If44dc7f5330f070c865b241d507082e253a92a4b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1641
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>