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

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

commit d2913a07d9037fe7aed4b7e680684163eaed6bc4 upstream.

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: syzbot+4bf62a7b1d0f4fdb7ae2@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20241120141104.7060-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>