hw/virtio/vhost-shadow-virtqueue: Silence GCC error "maybe-uninitialized"

GCC issues a false positive warning, resulting in build failure with -Werror:

  In file included from /usr/include/glib-2.0/glib.h:114,
                   from src/include/glib-compat.h:32,
                   from src/include/qemu/osdep.h:144,
                   from ../src/hw/virtio/vhost-shadow-virtqueue.c:10:
  In function ‘g_autoptr_cleanup_generic_gfree’,
      inlined from ‘vhost_handle_guest_kick’ at ../src/hw/virtio/vhost-shadow-virtqueue.c:292:42:
  /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘elem’ may be used uninitialized [-Werror=maybe-uninitialized]
     28 |   g_free (*pp);
        |   ^~~~~~~~~~~~
  ../src/hw/virtio/vhost-shadow-virtqueue.c: In function ‘vhost_handle_guest_kick’:
  ../src/hw/virtio/vhost-shadow-virtqueue.c:292:42: note: ‘elem’ was declared here
    292 |             g_autofree VirtQueueElement *elem;
        |                                          ^~~~
  cc1: all warnings being treated as errors

There is actually no problem since "elem" is initialized in both branches.
Silence the warning by initializig it with "NULL".

$ gcc --version
gcc (GCC) 12.2.0

Fixes: 9c2ab2f1ec ("vhost: stop transfer elem ownership in vhost_handle_guest_kick")
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220910151117.6665-1-shentey@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/virtio/vhost-shadow-virtqueue.c

@@ -289,7 +289,7 @@ static void vhost_handle_guest_kick(VhostShadowVirtqueue *svq)
         virtio_queue_set_notification(svq->vq, false);
 
         while (true) {
-            g_autofree VirtQueueElement *elem;
+            g_autofree VirtQueueElement *elem = NULL;
             int r;
 
             if (svq->next_guest_avail_elem) {