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.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/gus/gus_mem.c

@@ -44,7 +44,7 @@ static struct snd_gf1_mem_block *snd_gf1_mem_xalloc(struct snd_gf1_mem * alloc,
 			else
 				nblock->prev->next = nblock;
 			mutex_unlock(&alloc->memory_mutex);
-			return NULL;
+			return nblock;
 		}
 		pblock = pblock->next;
 	}