Remove no-longer-helpful reliance on fixed-size local array.

Remove no-longer-helpful reliance on fixed-size local array.

Coverity complained about this code, apparently because it uses a local
array of size FUNC_MAX_ARGS without a guard that the input argument list
is no longer than that.  (Not sure why it complained today, since this
code's been the same for a long time; possibly it re-analyzed everything
the List API change touched?)

Rather than add a guard, though, let's just get rid of the local array
altogether.  It was only there to avoid list_nth() calls, and those are
no longer expensive.