Make sure the filicide is justified

Using `^C` before a child is forked leads to a stack trace:
```
martin@mbp[master]$ bundle exec resque work
D, [2013-09-23T08:31:28.888182 #7529] DEBUG -- : resque-2.0.0.pre.1: Starting
D, [2013-09-23T08:31:28.888305 #7529] DEBUG -- : Registered signals
I, [2013-09-23T08:31:28.890092 #7529]  INFO -- : Running before_first_fork hooks with [#<Worker mbp.local:7529:default>]
^CI, [2013-09-23T08:31:32.827525 #7529]  INFO -- : Exiting...
/Users/martin/.rbenv/versions/1.9.3-p448/lib/ruby/gems/1.9.1/bundler/gems/resque-9274eff13859/lib/resque/worker.rb:173:in `shutdown!': undefined method `kill' for nil:NilClass (NoMethodError)
```