ALSA: compress: Remove pointless NULL check

stream can't be NULL here as we have just taken the address of it, so no
need for the check.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>