Reset num_active_touches on DeviceOff (#52496)
Reset num_active_touches on DeviceOff (#52496)

When disabling the device, reset num_active_touches to zero. Otherwise,
num_active_touches stays at the value it was on DeviceOff(). Future touches
add to that value until the index may go past priv->open_slots[].
That causes spurious memory corruption on touch ends.

And as of 55fc42e7c9b4948cadd4f98ef7b6a3b12e268e3e we ignore pre-existing
touches anyway.

Test-case:
- place num_touches fingers on the touchpad
- xinput disable <device>
- lift fingers
- xinput enable <device>
- place finger on device, num_active_touches is now (num_touches + 1)

X.Org Bug 52496 <http://bugs.freedesktop.org/show_bug.cgi?id=52496>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>
(cherry picked from commit a245d42f53096b1ae81e6702729f97ca508e5b5b)

Conflicts:
	src/synaptics.c

RsrchBoy/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics