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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/isa/gus/gus_mem.c

···
 			else
 				nblock->prev->next = nblock;
 			mutex_unlock(&alloc->memory_mutex);
-			return NULL;
+			return nblock;
 		}
 		pblock = pblock->next;
 	}