ALSA: usx2y: Don't call free_pages_exact() with NULL address

Unlike some other functions, we can't pass NULL pointer to
free_pages_exact().  Add a proper NULL check for avoiding possible
Oops.

Link: https://lore.kernel.org/r/20210517131545.27252-10-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>