ALSA: memalloc: Catch call with NULL snd_dma_buffer pointer

Although we've covered all calls with NULL dma buffer pointer, so far,
there may be still some else in the wild.  For catching such a case
more easily, add a WARN_ON_ONCE() in snd_dma_get_ops().

Fixes: 37af81c5 ("ALSA: core: Abstract memory alloc helpers")
Link: https://lore.kernel.org/r/20211105102103.28148-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/memalloc.c

@@ -631,6 +631,8 @@ static const struct snd_malloc_ops *dma_ops[] = {
 
 static const struct snd_malloc_ops *snd_dma_get_ops(struct snd_dma_buffer *dmab)
 {
+	if (WARN_ON_ONCE(!dmab))
+		return NULL;
 	if (WARN_ON_ONCE(dmab->dev.type <= SNDRV_DMA_TYPE_UNKNOWN ||
 			 dmab->dev.type >= ARRAY_SIZE(dma_ops)))
 		return NULL;