Server#finalize fails if the socket was unable to be created

We should only `close` if possible. 

```
Errno::EACCES: Permission denied - bind(2)
	/Users/tim/Projects/celluloid-io/lib/celluloid/io/tcp_server.rb:11:in `initialize'
	/Users/tim/Projects/celluloid-io/lib/celluloid/io/tcp_server.rb:11:in `new'
	/Users/tim/Projects/celluloid-io/lib/celluloid/io/tcp_server.rb:11:in `initialize'
	/Users/tim/Projects/reel/lib/reel/server.rb:13:in `new'
	/Users/tim/Projects/reel/lib/reel/server.rb:13:in `initialize'

NoMethodError: undefined method `close' for nil:NilClass
	/Users/tim/Projects/reel/lib/reel/server.rb:20:in `finalize'
	/Users/tim/Projects/celluloid/lib/celluloid/actor.rb:386:in `block in run_finalizer'
	/Users/tim/Projects/celluloid/lib/celluloid/tasks/task_fiber.rb:28:in `block in initialize'