shell: Make sure we still have touch or pointer focus when moving/resizing
shell: Make sure we still have touch or pointer focus when moving/resizing

It's possible to touch a surface to move it and let go before we get
to common_surface_move(), in which case we don't have a touch focus
when we get there.  For pointers, we could click a surface, but have the
surface go away before we get to common_surface_move(), in which
case the button count is non-zero, but we don't have a surface.

In either case we crash, so let's add a check to make sure we still
have a focus surface before we try to move it.

Closes: https://bugs.freedesktop.org/show_bug.cgi?id=73448

kwm81/weston,giucam/weston,kwm81/weston,mchalupa/weston,udoprog/weston,sir-murray/weston,Gnurou/weston,xorgy/weston,krezovic/weston,sir-murray/weston,jonnylamb/weston,Fantu/compositor-spice,udoprog/weston,eyolfson/weston,Gnurou/weston,eyolfson/weston,jonnylamb/weston,mchalupa/weston,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,giucam/weston,Fantu/compositor-spice,krezovic/weston,xorgy/weston