cpu_affinity() in Linux raise SystemError

[giampaolo]

...if passed [-1]

~/svn/psutil {master}$ python3 -c "import psutil; psutil.Process().cpu_affinity([-1])"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/giampaolo/svn/psutil/psutil/__init__.py", line 764, in cpu_affinity
    self._proc.cpu_affinity_set(list(set(cpus)))
  File "/home/giampaolo/svn/psutil/psutil/_pslinux.py", line 1067, in wrapper
    return fun(self, *args, **kwargs)
  File "/home/giampaolo/svn/psutil/psutil/_pslinux.py", line 1423, in cpu_affinity_set
    cext.proc_cpu_affinity_set(self.pid, cpus)
SystemError: <built-in function proc_cpu_affinity_set> returned NULL without setting an error