fix a memory leak in XRegisterIMInstantiateCallback

Analysis:

    _XimRegisterIMInstantiateCallback() opens an XIM and closes it using
    the internal function pointers, but the internal close function does
    not free the pointer to the XIM (this would be done in XCloseIM()).

Report/patch:

    Date: Mon, 03 Oct 2022 18:47:32 +0800
    From: Po Lu <luangruo@yahoo.com>
    To: xorg-devel@lists.x.org
    Subject: Re: Yet another leak in Xlib

    For reference, here's how I'm calling XRegisterIMInstantiateCallback:

    XSetLocaleModifiers ("");
    XRegisterIMInstantiateCallback (compositor.display,
                                    XrmGetDatabase (compositor.display),
                                    (char *) compositor.resource_name,
                                    (char *) compositor.app_name,
                                    IMInstantiateCallback, NULL);

    and XMODIFIERS is:

        @im=ibus

Signed-off-by: Thomas E. Dickey <dickey@invisible-island.net>