bug#54374: 29.0.50; previous-completion fails at beginning of completions buffer

[Juri Linkov]

X-Debbugs-Cc: Philip Kaludercic <philipk@posteo.net>

This is not reproducible in Emacs 28, so looks like a recent regression:

0. emacs -Q
1. M-x TAB
2. move point to the beginning of the *Completions* buffer
3. type S-TAB (<backtab>)

Debugger entered--Lisp error: (args-out-of-range 0 0)
  get-text-property(0 mouse-face)
  next-completion(-1)
  previous-completion(1)
  funcall-interactively(previous-completion 1)
  command-execute(previous-completion)
  completing-read-default("M-x " ...)
  read-extended-command()
  command-execute(execute-extended-command)