[PATCH] HID: wacom: fix an Oops caused by wacom_wac_finger_count_touches

We assumed all touch interfaces report touch data. But, Bamboo
and Intuos non-touch devices report express keys on touch
interface. We need to check touch_max before counting touches.

Reported-by: Tasos Sahanidis <redacted>
Signed-off-by: Ping Cheng <redacted>
---
Hi Jiri, this patch is based on for-4.1/wacom. The issue was
initially introduced in 4.1.
---
 drivers/hid/wacom_wac.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index 69c7df7..1cd80d2 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c

@@ -1072,6 +1072,9 @@ static int wacom_wac_finger_count_touches(struct wacom_wac *wacom)
 	int count = 0;
 	int i;
 
+	if (!touch_max)
+		return 0;
+
 	/* non-HID_GENERIC single touch input doesn't call this routine */
 	if ((touch_max == 1) && (wacom->features.type == HID_GENERIC))
 		return wacom->hid_data.tipswitch &&

-- 
1.9.1