Similar to
7c4f6cc145daf66cd9624f1e70dd5278241f5858, if we don't have
a background image from the desktop-shell client or the touch point
for some other reason doesn't hit a surface we trigger a
segfault as we try to deref the seat->touch->focus NULL pointer.
For touch, another problem could be fudgey calibration that ends up
giving touch coordinates outside the output space.
https://bugs.freedesktop.org/show_bug.cgi?id=72839
{
if (seat->touch->grab != &seat->touch->default_grab)
return;
+ if (seat->touch->focus == NULL)
+ return;
activate_binding(seat, data, seat->touch->focus->surface);
}