It is possible to run into an IndexError in the subprocess module's _communicate function.

```
    return run(
  File "subprocess.py", line 491, in run
  File "subprocess.py", line 1024, in communicate
  File "subprocess.py", line 1418, in _communicate
IndexError: list index out of range
```

The lines in question are: 

```
            if stdout is not None:
                stdout = stdout[0]
            if stderr is not None:
                stderr = stderr[0]
```

I believe this is due to the fact there is no safety checking to make sure that self._stdout_buff and self._stderr_buff have any content in them after being set to empty lists. 

The fix I suggest is to change the checks from `if stdout is not None` to simply `if stdout` to make sure it is a populated list. 

Example fixed code: 

```
            if stdout:
                stdout = stdout[0]
            if stderr:
                stderr = stderr[0]
```

If a more stringent check is required, we could expand that out to check type and length, such as `isinstance(stdout, list) and len(stdout) > 0:` but that is more then necessary currently.