Catch nil.split in parse_http_request

While troubleshooting with telnet I noticed that an invalid HTTP request results in the following trace.
To prevent the thread from dying - let's catch corner case and return a warning.

```
/usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:233:in `block in parse_http_request':

You have a nil object when you didn't expect it! (NoMethodError)
You might have expected an instance of Array.

The error occurred while evaluating nil.split
	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:226:in `each'
	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:226:in `each_with_index'
	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:226:in `parse_http_request'
	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:133:in `accept_and_process_next_request'
 	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler/thread_handler.rb:109:in `main_loop'
 	from /usr/lib/ruby/vendor_ruby/phusion_passenger/request_handler.rb:471:in `block (2 levels) in start_threads'
```