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

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

Semantic patch:

    @@
    expression e;
    @@

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

    @@
    expression e;
    @@

    - if (e != NULL) { free(e); }
    + free(e);

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I4d2d79a8beda4ca1ec7bacd5424c3c8e19d158f0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1698
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>