Since r325726, sysctl kern.proc.args on the current process no longer
works if the arguments are longer than the value of the sysctl
It looks like the new fast path only works properly if p->p_args is not
NULL. If p->p_args is NULL, this usually means that the arguments are
longer than ps_arg_cache_limit and must be retrieved from (pageable)
process memory and not wired p->p_args->ar_args. It is possible to
duplicate that piece of code as well but that is probably not worth it.
Therefore, skip the fast path if p->p_args is NULL.
This breaks building x11-toolkits/gnustep-gui with log