HID: multitouch: Fix fields from pen report ID being interpreted for multitouch

Fields like HID_DG_CONTACTCOUNT are outside of the physical collection,
but within the application collection and report ID. Make sure to catch
those fields that are not part of the mt_report_id and return 0 so they
can be processed with the pen. Otherwise, the wrong HID_DG_CONTACTCOUNT
will be applied to cc_index and result in dereferencing a null pointer in
mt_touch_report.

Signed-off-by: Brent Adam <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/hid-multitouch.c

···
 	/*
 	 * some egalax touchscreens have "application == HID_DG_TOUCHSCREEN"
 	 * for the stylus.
+	 * The check for mt_report_id ensures we don't process
+	 * HID_DG_CONTACTCOUNT from the pen report as it is outside the physical
+	 * collection, but within the report ID.
 	 */
 	if (field->physical == HID_DG_STYLUS)
+		return 0;
+	else if ((field->physical == 0) &&
+		 (field->report->id != td->mt_report_id) &&
+		 (td->mt_report_id != -1))
 		return 0;
 
 	if (field->application == HID_DG_TOUCHSCREEN ||