ALSA: gus: Fix erroneous memory allocation

snd_gf1_mem_xalloc() returns NULL incorrectly when the memory chunk is
allocated in the middle of the chain.  This patch corrects the return
value to treat it properly.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20211213132444.22385-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>