ALSA: hda: fix possible NULL dereference

After a for-loop was replaced by list_for_each_entry, see
Commit bbbc7e85 ("ALSA: hda - Allocate hda_pcm objects dynamically"),
Commit 751e2216

 ("ALSA: hda: fix possible null dereference"),
a possible NULL pointer dereference has been introduced; this patch adds
the NULL check on pcm->pcm, while leaving a potentially superfluous
check on pcm itself untouched.

Signed-off-by: Markus Osterhoff <linux-kernel@k-raum.org>
Cc: <stable@vger.kernel.org> #v4.1+
Signed-off-by: Takashi Iwai <tiwai@suse.de>