Setting the "last" member of struct roff_node was done at an extremely

Setting the "last" member of struct roff_node was done at an extremely
weird place.  Move it to the obviously correct place.
Surprisingly, this didn't cause any misformatting in the test suite
or in any base system manuals, but i cannot believe the code was
really correct for all conceivable input, and it would be very hard
to verify.  At the very least, it cannot have worked for man(7).