HID: wacom: Have wacom_tpc_irq guard against possible NULL dereference

The following Smatch complaint was generated in response to commit
2a6cdbdd ("HID: wacom: Introduce new 'touch_input' device"):

    drivers/hid/wacom_wac.c:1586 wacom_tpc_irq()
             error: we previously assumed 'wacom->touch_input' could be null (see line 1577)

The 'touch_input' and 'pen_input' variables point to the 'struct input_dev'
used for relaying touch and pen events to userspace, respectively. If a
device does not have a touch interface or pen interface, the associated
input variable is NULL. The 'wacom_tpc_irq()' function is responsible for
forwarding input reports to a more-specific IRQ handler function. An
unknown report could theoretically be mistaken as e.g. a touch report
on a device which does not have a touch interface. This can be prevented
by only calling the pen/touch functions are called when the pen/touch
pointers are valid.

Fixes: 2a6cdbdd ("HID: wacom: Introduce new 'touch_input' device")
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -1571,10 +1571,15 @@ static int wacom_tpc_irq(struct wacom_wac *wacom, size_t len)
 {
 	unsigned char *data = wacom->data;
 
-	if (wacom->pen_input)
+	if (wacom->pen_input) {
 		dev_dbg(wacom->pen_input->dev.parent,
 			"%s: received report #%d\n", __func__, data[0]);
-	else if (wacom->touch_input)
+
+		if (len == WACOM_PKGLEN_PENABLED ||
+		    data[0] == WACOM_REPORT_PENABLED)
+			return wacom_tpc_pen(wacom);
+	}
+	else if (wacom->touch_input) {
 		dev_dbg(wacom->touch_input->dev.parent,
 			"%s: received report #%d\n", __func__, data[0]);
 
@@ -1585,9 +1590,6 @@ static int wacom_tpc_irq(struct wacom_wac *wacom, size_t len)
 		case WACOM_PKGLEN_TPC2FG:
 			return wacom_tpc_mt_touch(wacom);
 
-	case WACOM_PKGLEN_PENABLED:
-		return wacom_tpc_pen(wacom);
-
 		default:
 			switch (data[0]) {
 			case WACOM_REPORT_TPC1FG:
@@ -1600,8 +1602,7 @@ static int wacom_tpc_irq(struct wacom_wac *wacom, size_t len)
 			case WACOM_REPORT_TPCMT2:
 				return wacom_mt_touch(wacom);
 
-		case WACOM_REPORT_PENABLED:
-			return wacom_tpc_pen(wacom);
+			}
 		}
 	}