bug#56273: [REGRESSION] (x-clipboard-yank) pastes from kill-ring when `select-enable-clipboard` is t

[Konstantin Kharlamov]

After upgrade to latest development version at:

      05fe0faed4: "make clean" etc. problem now fixed on Solaris.

I found out that using (x-clipboard-yank) when `select-enable-
clipboard` is set to t randomly pastes stuff that is not in system
clipboard. I don't know exact commit that broke it, am in process of
bisecting right now. Latest one working that I found so far is
7055104358ad51b8c49e120851ee5d4d3c909e6d.


# Steps to reproduce

1. Create /tmp/.emacs file with the following content:

    (setq select-enable-clipboard nil
          inhibit-startup-screen t)

    (defun clipboard-yank-test ()
      (interactive)
      (let ((select-enable-clipboard t))
        (x-clipboard-yank)))

    (global-set-key "\C-y" 'clipboard-yank-test)
2. Launch `HOME=/tmp/ emacs`
3. Outside Emacs copy a word "test"
4. In Emacs ensure that `C-y` pastes word "test"
5. In Emacs, delete a different word in the *scratch* buffer, e.g. a
"create"
6. In Emacs, press C-y to paste current clipboard content

## Expected

A word "test" will be inserted as this is the one that is in the system
clipboard.

Note: the word "create" didn't replace content in the clipboard,
because global value of select-enable-clipboard is nil. So it still has
"test", and the C-y was expected to paste "test" as well due to
`select-enable-clipboard` being temporarily set to `t` in clipboard-
yank-test.

## Actual

The word you deleted in *scratch* buffer will be inserted instead of
"test".