ALSA: Fix card refcount unbalance

There are uncovered cases whether the card refcount introduced by the
commit a0830dbd isn't properly increased or decreased:
- OSS PCM and mixer success paths
- When lookup function gets NULL

This patch fixes these places.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=50251

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 6e8872d..f9ddecf 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -2122,7 +2122,8 @@
 	pcm = snd_lookup_minor_data(iminor(inode),
 				    SNDRV_DEVICE_TYPE_PCM_PLAYBACK);
 	err = snd_pcm_open(file, pcm, SNDRV_PCM_STREAM_PLAYBACK);
-	snd_card_unref(pcm->card);
+	if (pcm)
+		snd_card_unref(pcm->card);
 	return err;
 }
 
@@ -2135,7 +2136,8 @@
 	pcm = snd_lookup_minor_data(iminor(inode),
 				    SNDRV_DEVICE_TYPE_PCM_CAPTURE);
 	err = snd_pcm_open(file, pcm, SNDRV_PCM_STREAM_CAPTURE);
-	snd_card_unref(pcm->card);
+	if (pcm)
+		snd_card_unref(pcm->card);
 	return err;
 }