ui/gtk: making dmabuf NULL when it's released.

Set vc->gfx.guest_fb.dmabuf to NULL to prevent any further access
to it after the dmabuf is released.

v2: move declaration of vc inside ifdef

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20230627005316.5627-1-dongwon.kim@intel.com>

ui/gtk.c

@@ -583,7 +583,12 @@ static void gd_gl_release_dmabuf(DisplayChangeListener *dcl,
                                  QemuDmaBuf *dmabuf)
 {
 #ifdef CONFIG_GBM
+    VirtualConsole *vc = container_of(dcl, VirtualConsole, gfx.dcl);
+
     egl_dmabuf_release_texture(dmabuf);
+    if (vc->gfx.guest_fb.dmabuf == dmabuf) {
+        vc->gfx.guest_fb.dmabuf = NULL;
+    }
 #endif
 }