ALSA: core: fix NULL checking in snd_pcm_plug_client_size()

The dereference snd_pcm_plug_stream(plug) should come after the NULL
check snd_BUG_ON(!plug).

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>