gfx/common/x11: NULL-guard xss_screensaver_inhibit()

Found by the new ASan+UBSan CI workflow's headless SDL2
imageviewer smoke (b9777c8 + d967813).  Run #2 of the smoke
(after the v12 xvideo->sdl2 pivot) caught a SEGV at process
startup:

   AddressSanitizer:DEADLYSIGNAL
   ==9537==ERROR: AddressSanitizer: SEGV on unknown address
   0x000000000968.  The signal is caused by a READ memory
   access.

   #0 XQueryExtension      (libX11.so.6+0x361f6)
   #1 XInitExtension       (libX11.so.6)
   #2 XextAddDisplay       (libXext.so.6)
   #3 XScreenSaverQueryExtension (libXss.so.1+0x1565)
   #4 x11_suspend_screensaver
   #5 video_driver_init_internal
   #6 drivers_init
   #7 retroarch_main_init

The address 0x968 is a small offset off NULL -- libX11's
XQueryExtension dereferencing the Display * argument before
checking it for null.

Root cause:
gfx/common/x11_common.c::xss_screensaver_inhibit() was called
via x11_suspend_screensaver() with g_x11_dpy == NULL.  The flow:

  1. SDL2 video driver inits.
  2. sdl2_set_handles() in gfx/common/sdl2_common.c:54 calls
     video_driver_display_type_set(RARCH_DISPLAY_X11) so the
     display *type* gate matches X11, and routes the SDL-owned
     X Display through video_driver_display_set() at line 55.
  3. The file-scope `g_x11_dpy` in x11_common.c:66 stays at its
     initial NULL value.  It's only assigned at line 795 inside
     XOpenDisplay() in the xvideo / GL / X11-direct init paths,
     which the SDL2 driver doesn't take.
  4. drivers_init() -> video_driver_init_internal() ->
     x11_suspend_screensaver() -- line 286 gates on display_type
     == RARCH_DISPLAY_X11 (passes; SDL2 set it), then at lines
     289-292 tries dbus_suspend_screensaver() if HAVE_DBUS, then
     at line 293 calls xss_screensaver_inhibit(g_x11_dpy,
     enable) -- with g_x11_dpy == NULL.
  5. xss_screensaver_inhibit() at line 227 calls
     XScreenSaverQueryExtension(NULL, ...) -> libX11
     dereferences -> SEGV.

Why hasn't this been hit before?
Most desktop builds set HAVE_DBUS -- pkg-config finds dbus-1 on
basically every modern Linux distro -- so the
dbus_suspend_screensaver() call at line 290 short-circuits and
returns true before the xss path is reached.  The CI runner
doesn't apt-install libdbus-1-dev (HAVE_DBUS off) but libxss1 is
pulled in transitively by xvfb / x11-utils (HAVE_XSCRNSAVER on
at link time), exposing the corner.

A symmetric defensive check already exists at line 255 in
xdg_screensaver_inhibit():

   if (g_x11_dpy && g_x11_win)
      ...

The xss path was the missing-guard one.

* gfx/common/x11_common.c

  Add a `if (!dpy) return false;` guard at the top of
  xss_screensaver_inhibit().  Applies only to the
  HAVE_XSCRNSAVER branch; the no-op stub at line 237 already
  discards `dpy` with `(void) dpy;` and returns false.

  Inline comment explaining the failure mode points at the
  relevant call sites in sdl2_common.c so a future maintainer
  can trace the invariant without re-deriving it.

Verified locally:
  - The added check returns false for NULL dpy without
    invoking libX11 (matches the existing "extension not
    present" return contract).  Non-NULL dpy paths are
    unchanged.
  - xss_screensaver_inhibit() is `static`, single caller (line
    311 in x11_suspend_screensaver) -- no other reachable code
    paths to audit.

Cannot verify locally that this resolves the CI SEGV without
rebuilding retroarch with the same apt set the runner has --
the next CI run is the experiment.  Expected outcome:
xss_screensaver_inhibit() returns false harmlessly, the
function falls through to the xdg_screensaver path (which has
its own NULL guard), and the SDL2 smoke proceeds into
retro_run.

Author: LibretroAdmin

gfx/common/x11_common.c

@@ -224,6 +224,24 @@ void x11_set_window_attr(Display *dpy, Window win)
 static bool xss_screensaver_inhibit(Display *dpy, bool enable)
 {
     int dummy, min, maj;
+    /* Guard against being called with a NULL Display.  This
+     * happens with the SDL2 video driver under HAVE_X11 +
+     * HAVE_XSCRNSAVER builds without HAVE_DBUS: SDL2's
+     * sdl2_set_handles() in gfx/common/sdl2_common.c sets the
+     * display *type* to RARCH_DISPLAY_X11 (so the gate in
+     * x11_suspend_screensaver passes) and routes the SDL-owned
+     * X Display through video_driver_display_set(), but the
+     * file-scope g_x11_dpy here stays at its initial NULL --
+     * it's only assigned in the xvideo / GL / X11-direct init
+     * paths.  libX11's XQueryExtension() then SEGVs at a tiny
+     * offset off the NULL display pointer.  Most desktop builds
+     * never hit this because HAVE_DBUS is on and
+     * dbus_suspend_screensaver() short-circuits before this
+     * line; surfaced by the ASan+UBSan CI workflow's headless
+     * SDL2 smoke (b9777c8 + d967813), where dbus-1 isn't
+     * apt-installed but libXss is. */
+    if (!dpy)
+       return false;
     if (       !XScreenSaverQueryExtension(dpy, &dummy, &dummy)
             || !XScreenSaverQueryVersion(dpy, &maj, &min)
             || (maj < 1)