Fix botched check for being off the mesh...

Marlin/src/feature/bedlevel/ubl/ubl.h

        * UBL_Z_RAISE_WHEN_OFF_MESH is specified, that value is returned.
        */
       #ifdef UBL_Z_RAISE_WHEN_OFF_MESH
-        if (!WITHIN(rx0, 0, GRID_MAX_POINTS_X - 1) || !WITHIN(ry0, 0, GRID_MAX_POINTS_Y - 1))
+        if (!WITHIN(rx0, MESH_MIN_X, MESH_MAX_X) || !WITHIN(ry0, MESH_MIN_Y, MESH_MAX_Y))
           return UBL_Z_RAISE_WHEN_OFF_MESH;
       #endif