ALSA: pcm: potential uninitialized return values

Smatch complains that "tmp" can be uninitialized if we do a zero size
write.

Fixes: 02a5d692

 ("ALSA: pcm: Avoid potential races between OSS ioctls and read/write")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>