Fix portal player y pos being off by 1 node sometimes

[Desour]

There were some changes in how vector.round rounds: https://github.com/minetest/minetest/pull/10803 => It now rounds y down if at y<0 (i.e. in nether).

The new pos uses the highest node that contains the feet.

(collisionbox[2] is 0 by default in mtg, btw.)