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:  <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>