Fix default inherited stdout/stderr when PosixSpawnProcess does not have an @io.

`PosixSpawnProcess` only set io streams if `@io` exists which is not the case if the process creator never calls `#io`. Therefor the stdout and stderr seems to be inherited by default.

This fix changes the if conditions to use the `io` getter method that provides a lazy initialization for Unix::IO.

I've encountered that when using selenium webdriver along other processes with posix spawn. Firefox started to print all log output and internal error messages in my test runner output. 
The fix works locally for me and specs are passing. I haven't found any `PosixSpawnProcess` specs - so no idea where to add a spec.