Fix parse error and Passenger Enterprise compatibility

Pull request https://github.com/deivid-rodriguez/byebug/pull/42 broke Passenger Enterprise compatibility. Passenger Enterprise calls `Byebug.start_server("127.0.0.1", 0)` to allow the OS to take a randomly available port. Because of the `port+1` code in pull request 42, the code would try to start the control server on port 1, which would usually fail. This pull request fixes it.

I also encountered a parse error in frame.rb so I fixed that too.