ALSA: PCM: Fix NULL dereference at mmap checks

The recent refactoring of mmap handling caused Oops on some devices
that don't use the standard memory allocations.  This patch addresses
it by allowing snd_dma_buffer_mmap() helper to receive the NULL
pointer dmab argument (and return an error appropriately).

Fixes: a202bd1a ("ALSA: core: Move mmap handler into memalloc ops")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211107163911.13534-1-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/memalloc.c

@@ -183,8 +183,11 @@ EXPORT_SYMBOL_GPL(snd_devm_alloc_dir_pages);
 int snd_dma_buffer_mmap(struct snd_dma_buffer *dmab,
 			struct vm_area_struct *area)
 {
-	const struct snd_malloc_ops *ops = snd_dma_get_ops(dmab);
+	const struct snd_malloc_ops *ops;
 
+	if (!dmab)
+		return -ENOENT;
+	ops = snd_dma_get_ops(dmab);
 	if (ops && ops->mmap)
 		return ops->mmap(dmab, area);
 	else