ALSA: xen-front: freeing an error pointer

kfree() doesn't accept error pointers so I've set "str" to NULL on these
paths.

Fixes: fd3b3604

 ("ALSA: xen-front: Read sound driver configuration from Xen store")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>