sound: Drain buffer selinfo in sndbuf_free()

Prevent a use-after-free in kern_poll() by making sure the buffer's
selinfo is drained. This is required for a subsequent patch that
implements asynchronous audio device detach.

Reported by:	KASAN
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D44544

sys/dev/sound/pcm/buffer.c

@@ -147,6 +147,7 @@ sndbuf_free(struct snd_dbuf *b)
 		} else
 			free(b->buf, M_DEVBUF);
 	}
+	seldrain(sndbuf_getsel(b));
 
 	b->tmpbuf = NULL;
 	b->shadbuf = NULL;