Fix invalid argument bug when running in Python3.3

reset_signals() will cause OSError: [Errno 22] Invalid argument when running celery in Python 3.3 on OS X 10.8.3.

It's because signal.getsignal(signal.SIGKILL) and signal.getsignal(signal.SIGSTOP) will return None value.

I've tested the change on my machine, and it fixes the bug.