[ALSA] no need to check pointers passed to vfree() for NULL

Modules: Digigram VX core,USB generic driver

There's no need to check pointers passed to vfree() for NULL.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>