subsurface_get_root_coords: break on NULL
subsurface_get_root_coords: break on NULL

It is possible for `wlr_surface_is_subsurface` to return true, but
`wlr_surface_from_wlr_surface` to be NULL. This adds a NULL check to the
value returned by `wlr_surface_from_wlr_surface` and breaks out of the
while loop in `subsurface_get_root_coords`.

ascent12/sway,1ace/sway,1ace/sway,SirCmpwn/sway,1ace/sway,ascent12/sway,ascent12/sway