amx completion includes commands that are not defined anymore

[bcc32]

amx completion includes history elements that may not correspond to actually commands (i.e., commandp returns nil for them).

Invoking such commands always produces an error, so I think they should not be suggested even if they appear in history.

This can be confusing if:

• If a package was updated and the command no longer exists
• If the command was defined in some package but was not autoloaded, and that package isn't loaded at startup

reproduction steps

1. emacs (no -q because that disables amx). Assume that the init file enables amx-mode.

2. Define some command, or load a file which does:

   (defun this-is-a-deleted-command ()
     (interactive)
     (message "This command will be deleted in a future update of this package."))
   

3. M-x this-is-a-deleted-command

4. kill-emacs and restart Emacs

5. M-x this-is contains this-is-a-deleted-command despite that command not being defined.

6. Press RET. This produces a type error:

   Wrong type argument: commandp, this-is-a-deleted-command