Eliminate the possibility of passing NULL to snd_card_free().

Thanks to Takashi Iwai for reviewing and pointing this out.

This work was sponsored by ONELAN Limited.

Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>