[ALSA] Fix gus_pcm dereference before NULL

Modules: GUS Library

The NULL check of substream is simply superfluous. It is
guaranteed to receive non-NULL substream. Thanks Takashi.

Coverity bug #861

Signed-off-by: Eugene Teo <eugene.teo@eugeneteo.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>