Fix: check for NULL in destroy_pretty_data

Fix: check for NULL in destroy_pretty_data

In pretty_init, if create_pretty fails, we go to the `error` label and
call destroy_pretty_data.  However, destroy_pretty_data does not accept
a NULL pointer.

My understanding is that our "destroy" functions generally check for a
NULL pointer, and return early in that case.

Change destroy_pretty_data to do just that.

Fixes Coverity:

    *** CID 1405908:  Null pointer dereferences  (FORWARD_NULL)
    /src/plugins/text/pretty/pretty.c: 588 in pretty_init()
    582     	bt_self_component_set_data(self_comp, pretty);
    583
    584     	status = BT_COMPONENT_CLASS_INITIALIZE_METHOD_STATUS_OK;
    585     	goto end;
    586
    587     error:
    >>>     CID 1405908:  Null pointer dereferences  (FORWARD_NULL)
    >>>     Passing null pointer "pretty" to "destroy_pretty_data", which dereferences it.
    588     	destroy_pretty_data(pretty);
    589
    590     end:
    591     	return status;

Reported-by: Coverity
Change-Id: I65ec6f8b2781c0a3a1166a1e68da1f47dc2da452
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2178
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>