When provided an invalid display name CLI argument, xpra throws IndexError: string index out of range

[audibleptr]

Describe the bug
When provided an invalid display name CLI argument (which may happen accidentally), xpra throws IndexError: string index out of range.

This may not be strictly a bug, but it would be better if xpra provided a more easily understandable error message to the user.

To Reproduce

xpra stop :

System Information:

• Server OS:

  # uname -a
  Linux kworkstation-10-20230406.localdomain 5.15.104-un-def-alt1 #1 SMP PREEMPT Tue Mar 28 17:27:40 UTC 2023 x86_64 GNU/Linux
  

• Client OS: n/a (not used)
• Xpra Server Version:

  # rpm -qa | grep xpra
  xpra-4.4.4-alt2.x86_64
  

• Xpra Client Version: n/a (not used)

Additional context

Actual result:

xpra main error:
Traceback (most recent call last):
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 144, in main
    return run_mode(script_file, cmdline, err, options, args, mode, defaults)
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 453, in run_mode
    return do_run_mode(script_file, cmdline, error_cb, options, args, mode, defaults)
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 500, in do_run_mode
    return run_stopexit(mode, error_cb, options, args, cmdline)
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 2776, in run_stopexit
    display_desc = pick_display(error_cb, opts, extra_args, cmdline)
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 816, in pick_display
    return do_pick_display(dotxpra, error_cb, opts, extra_args, cmdline)
  File "/usr/lib64/python3/site-packages/xpra/scripts/main.py", line 849, in do_pick_display
    return parse_display_name(error_cb, opts, extra_args[0], cmdline, find_session_by_name=find_session_by_name)
  File "/usr/lib64/python3/site-packages/xpra/scripts/parsing.py", line 420, in parse_display_name
    while display_name[pos]=="/" and scount<2:
IndexError: string index out of range

Expected result: An error that is more easily understandable to the user.

[totaam]

Thanks, the commit above fixes that.

[totaam]

I had fat fingered the commit, fixed properly now.