usb: gadget: u_audio: Clear uac pointer when freed.

This prevents use of a stale pointer if functions are called after
g_cleanup that shouldn't be. This doesn't fix any races, but converts
a possibly silent kernel memory corruption into an obvious NULL pointer
dereference report.

Fixes: eb9fecb9e69b ("usb: gadget: f_uac2: split out audio core")
Signed-off-by: Chris Wulff <[e-mail]>
Link: https://lore.kernel.org/stable/CO1PR17MB54194226DA08BFC9EBD8C163E1172%40CO1PR17MB5419.namprd17.prod.outlook.com
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/u_audio.c

···
 		return;
 
 	uac = g_audio->uac;
+	g_audio->uac = NULL;
+
 	card = uac->card;
 	if (card)
 		snd_card_free_when_closed(card);