HID: wacom: Avoid using stale array indicies to read contact count

commit 20f3cf5f860f9f267a6a6e5642d3d0525edb1814 upstream.

If we ever see a touch report with contact count data we initialize
several variables used to read the contact count in the pre-report
phase. These variables are never reset if we process a report which
doesn't contain a contact count, however. This can cause the pre-
report function to trigger a read of arbitrary memory (e.g. NULL
if we're lucky) and potentially crash the driver.

This commit restores resetting of the variables back to default
"none" values that were used prior to the commit mentioned
below.

Link: https://github.com/linuxwacom/input-wacom/issues/276


Fixes: 003f50ab (HID: wacom: Update last_slot_field during pre_report phase)
CC: stable@vger.kernel.org
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>