bug#56185: 29.0.50; imenu with imenu-use-popup-menu fails for *Rescan* item

[Visuwesh <visuweshm@gmail.com>]

When imenu-use-popup-menu is non-nil and one selects the *Rescan* item,
imenu fails with the following backtrace,

    Debugger entered--Lisp error: (error "Invalid `key-parse' syntax: %S" ["*Rescan*"])
      lookup-key((keymap "sh" ("*Rescan*" "*Rescan*" . #f(compiled-function () (interactive nil) #<bytecode -0x1823bf90e9c446ac>)) ("x" "x" . #f(compiled-function () (interactive nil) #<bytecode -0x19fbbb6b7dca5348>)) ("y" "y" . #f(compiled-function () (interactive nil) #<bytecode -0x19fbbb6b7da17348>))) ["*Rescan*"])
      popup-menu((keymap "sh" ("*Rescan*" "*Rescan*" . #f(compiled-function () (interactive nil) #<bytecode -0x1823bf90e9c446ac>)) ("x" "x" . #f(compiled-function () (interactive nil) #<bytecode -0x19fbbb6b7dca5348>)) ("y" "y" . #f(compiled-function () (interactive nil) #<bytecode -0x19fbbb6b7da17348>))) 105)
      imenu--mouse-menu((("*Rescan*" . -99) ("x" . #<marker at 1 in sh>) ("y" . #<marker at 7 in sh>)) 105)
      imenu-choose-buffer-index()
      byte-code("\300 C\207" [imenu-choose-buffer-index] 1)
      call-interactively(imenu nil nil)
      command-execute(imenu)

Sometimes, this is also observed with an item other than *Rescan*:

    Debugger entered--Lisp error: (error "Invalid `key-parse' syntax: %S" ["x (def)"])
      lookup-key((keymap "sh" ("*Rescan*" "*Rescan*" . #f(compiled-function () (interactive nil) #<bytecode -0x1823bf90e9c446ac>)) ("x (def)" "x (def)" . #f(compiled-function () (interactive nil) #<bytecode -0x19471a772db71348>)) ("y (def)" "y (def)" . #f(compiled-function () (interactive nil) #<bytecode -0x194701872ad91348>))) ["x (def)"])
      popup-menu((keymap "sh" ("*Rescan*" "*Rescan*" . #f(compiled-function () (interactive nil) #<bytecode -0x1823bf90e9c446ac>)) ("x (def)" "x (def)" . #f(compiled-function () (interactive nil) #<bytecode -0x19471a772db71348>)) ("y (def)" "y (def)" . #f(compiled-function () (interactive nil) #<bytecode -0x194701872ad91348>))) 105)
      imenu--mouse-menu((("*Rescan*" . -99) ("x (def)" . #<marker at 1 in sh>) ("y (def)" . #<marker at 24 in sh>)) 105)
      imenu-choose-buffer-index()
      byte-code("\300 C\207" [imenu-choose-buffer-index] 1)
      call-interactively(imenu nil nil)
      command-execute(imenu)

this is in a python-mode buffer containing the following text,

    def x():
        return y

    def y():
        return x

No such problem is seen when adding the index to the menubar using M-x
imenu-add-menubar-index however.

In GNU Emacs 29.0.50 (build 19, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2022-06-21 built on astatine
Repository revision: be35c92c90d455739a6ff9d4beefa2b35d044852
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101003
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --with-modules --with-sound=alsa --with-x-toolkit=lucid
 --with-json --without-xaw3d --without-gconf --without-libsystemd
 --with-x --without-cairo'

Configured features:
ACL DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG JSON
LIBOTF LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS WEBP X11 XDBE XFT
XIM XINPUT2 XPM LUCID ZLIB

Important settings:
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  value of $LC_NUMERIC: ta_IN.UTF-8
  value of $LANG: en_GB.UTF-8
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Major mode: ELisp/l

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Load-path shadows:
None found.

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Memory information:
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