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>

sound/core/oss/pcm_plugin.c

@@ -199,12 +199,13 @@ int snd_pcm_plugin_free(struct snd_pcm_plugin *plugin)
 snd_pcm_sframes_t snd_pcm_plug_client_size(struct snd_pcm_substream *plug, snd_pcm_uframes_t drv_frames)
 {
 	struct snd_pcm_plugin *plugin, *plugin_prev, *plugin_next;
-	int stream = snd_pcm_plug_stream(plug);
+	int stream;
 
 	if (snd_BUG_ON(!plug))
 		return -ENXIO;
 	if (drv_frames == 0)
 		return 0;
+	stream = snd_pcm_plug_stream(plug);
 	if (stream == SNDRV_PCM_STREAM_PLAYBACK) {
 		plugin = snd_pcm_plug_last(plug);
 		while (plugin && drv_frames > 0) {