ALSA: emux: stop if copy_from_user() fails

If we can't fill the "patch" struct because "count" is too small (it can
be as low as 4 bytes) or because copy_from_user() failed, then just
return instead of using unintialized data.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>