ALSA: sb: Fix potential double-free of CSP mixer elements

stable inclusion
from linux-4.19.198
commit 9ec6184908c650f261eb09fdb18a513150ea1ce7

--------------------------------

[ Upstream commit c305366a ]

snd_sb_qsound_destroy() contains the calls of removing the previously
created mixer controls, but it doesn't clear the pointers.  As
snd_sb_qsound_destroy() itself may be repeatedly called via ioctl,
this could lead to double-free potentially.

Fix it by clearing the struct fields properly afterwards.

Link: https://lore.kernel.org/r/20210608140540.17885-4-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

sound/isa/sb/sb16_csp.c

@@ -1086,10 +1086,14 @@ static void snd_sb_qsound_destroy(struct snd_sb_csp * p)
 	card = p->chip->card;	
 	
 	down_write(&card->controls_rwsem);
-	if (p->qsound_switch)
+	if (p->qsound_switch) {
 		snd_ctl_remove(card, p->qsound_switch);
-	if (p->qsound_space)
+		p->qsound_switch = NULL;
+	}
+	if (p->qsound_space) {
 		snd_ctl_remove(card, p->qsound_space);
+		p->qsound_space = NULL;
+	}
 	up_write(&card->controls_rwsem);
 
 	/* cancel pending transfer of QSound parameters */