Hi,

There is an unexpected error message happening with function
`python-shell-completion-at-point`.
> Wrong type argument: processp, nil
It happened because the function does NOT check the process param.
The patch will check the param and display a user error message gracefully.

Steps to reproduce the issue.
1. $ emacs -q /tmp/a.py # start emacs and open a python source file
2. type "C-c C-p" (or M-x run-python) to start a python shell
3. type exit() to stop the python process but the buffer still exists
4. M-x python-shell-completion-at-point  will get the error message

Please help review the patch. Thanks. Regards