[PATCH] Fix formatting in arena.c

[Roland McGrath]

Actually in this case (and some others) it's not an "obvious" case at
all.  That is code we originally imported from elsewhere, and so we
left its formatting alone so as to minimize gratuitous diffs from the
original.  In the case of malloc, we have diverged quite a lot from
the original versions we started with and may well never try to track
any changes from those, so it matters less than it might.  But
regardless, I don't think we want to reformat the malloc code any time soon.