polar-coordinates: Fix warning about use of uninitialized variables

This is ideally fixed by using something like:

  if (inside)
    {
      ud = rx;
      vd = ry;
    }

... but because the gegl_unmap() macro is also used in the
gegl_sampler_compute_scale() macro, the warning persists due to the
variables passed as ud, vd being used there.