Fix: lib: add NULL check for destroy_listeners in destroy_component_class

Fix: lib: add NULL check for destroy_listeners in destroy_component_class

By doing this change, to simulate an allocation failure:

    @@ -99,6 +99,8 @@ int bt_component_class_init(struct bt_component_class *class,
     	}

     	class->description = g_string_new(NULL);
    +	g_string_free(class->description, 1);
    +	class->description = NULL;
     	if (!class->description) {
     		BT_LIB_LOGE_APPEND_CAUSE("Failed to allocate a GString.");
     		goto error;

... and trying to read a trace, we get this crash:

    ==2788787==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008 (pc 0x7f779199cd2b bp 0x7ffd04f1a300 sp 0x7ffd04f1a2c0 T0)
    ==2788787==The signal is caused by a READ memory access.
    ==2788787==Hint: address points to the zero page.
        #0 0x7f779199cd2b in destroy_component_class /home/simark/src/babeltrace/src/lib/graph/component-class.c:39
        #1 0x7f779199ca19 in bt_object_put_ref_no_null_check /home/simark/src/babeltrace/src/lib/object.h:307
        #2 0x7f779199cb85 in bt_object_put_ref /home/simark/src/babeltrace/src/lib/object.h:335
        #3 0x7f779199d57f in bt_component_class_init /home/simark/src/babeltrace/src/lib/graph/component-class.c:131

This is because destroy_component_class can be called before the
destroy_listeners field has been initialized.  Add a check for
destroy_listeners to be non-NULL in destroy_component_class.

Change-Id: I3616f608f05e2abed4a85a62bebe05b1efedeb2b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7185
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>