ALSA: pcm: Add sanity NULL check for the default mmap fault handler

A driver might allow the mmap access before initializing its
runtime->dma_area properly. Add a proper NULL check before passing to
virt_to_page() for avoiding a panic.

Reported-by: [e-mail]
Cc: <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/pcm_native.c

···
 		return VM_FAULT_SIGBUS;
 	if (substream->ops->page)
 		page = substream->ops->page(substream, offset);
-	else if (!snd_pcm_get_dma_buf(substream))
+	else if (!snd_pcm_get_dma_buf(substream)) {
+		if (WARN_ON_ONCE(!runtime->dma_area))
+			return VM_FAULT_SIGBUS;
 		page = virt_to_page(runtime->dma_area + offset);
-	else
+	} else
 		page = snd_sgbuf_get_page(snd_pcm_get_dma_buf(substream), offset);
 	if (!page)
 		return VM_FAULT_SIGBUS;