shell: Only assign focus on touch if there is a touch focus surface

shell: Only assign focus on touch if there is a touch focus surface

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