ALSA: firewire-lib: suppress kernel warnings when releasing uninitialized stream data

When any of AMDTP stream data are not initialized and private data is
going to be released, WARN_ON() in amdtp_stream_destroy() is hit and
dump messages. This may take users irritated.

This commit fixes the bug to skip releasing when it's not initialized.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/amdtp-stream.c

@@ -102,6 +102,10 @@ EXPORT_SYMBOL(amdtp_stream_init);
  */
 void amdtp_stream_destroy(struct amdtp_stream *s)
 {
+	/* Not initialized. */
+	if (s->protocol == NULL)
+		return;
+
 	WARN_ON(amdtp_stream_running(s));
 	kfree(s->protocol);
 	mutex_destroy(&s->mutex);