Do not check `NULL` to call g_free(): g_free() accepts `NULL`

Do not check `NULL` to call g_free(): g_free() accepts `NULL`

Semantic patch:

    @@
    expression e;
    @@

    - if (e) { g_free(e); }
    + g_free(e);

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ifbbca48f04cc198543aaa5d06d3b906b32fdf137
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1684
Tested-by: jenkins <jenkins@lttng.org>