ALSA: pcm: Fix stream lock usage in snd_pcm_period_elapsed()

If the nullity check for `substream->runtime` is outside of the lock
region, it is possible to have a null runtime in the critical section
if snd_pcm_detach_substream is called right before the lock.

Signed-off-by: paulhsia <paulhsia@chromium.org>
Link: https://lore.kernel.org/r/20191112171715.128727-2-paulhsia@chromium.org


Signed-off-by: Takashi Iwai <tiwai@suse.de>