[ALSA] Fix bug in snd-usb-usx2y's usX2Y_pcms_lock_check()

Fix bug in snd-usb-usx2y's usX2Y_pcms_lock_check()
substream can be NULL......
in mainline, bug was introduced by:
2006-06-22  [ALSA] Add O_APPEND flag support to PCM
Signed-off-by: NKarsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@suse.cz>

sound/usb/usx2y/usx2yhwdeppcm.c

@@ -632,7 +632,7 @@ static int usX2Y_pcms_lock_check(struct snd_card *card)
 		for (s = 0; s < 2; ++s) {
 			struct snd_pcm_substream *substream;
 			substream = pcm->streams[s].substream;
-			if (SUBSTREAM_BUSY(substream))
+			if (substream && SUBSTREAM_BUSY(substream))
 				err = -EBUSY;
 		}
 	}