wayland: Fix null pointer deference in meta_get_first_subsurface_node.

In fcfe90aa, multiple for loops were replaced with
META_WAYLAND_SURFACE_FOREACH_SUBSURFACE.

However, this substitution was not side-effect free, and introduced a
null-pointer dereference risk as shown in the example below:

Old:

    for (n = g_node_first_child (surface->subsurface_branch_node);
         n;
         n = g_node_next_sibling (n))
      {
        if (G_NODE_IS_LEAF (n))
          continue;

        meta_wayland_surface_update_outputs_recursively (n->data);
      }

n is checked for NULL during each loop in the condition expression.
Therefore, when `G_NODE_IS_LEAF (n)` is called, `n` is guaranteed not to
be NULL. Note also that g_node_first_child is also NULL-safe since it
performs a NULL check internally.

New:

    META_WAYLAND_SURFACE_FOREACH_SUBSURFACE (surface, subsurface_surface)
      meta_wayland_surface_update_outputs_recursively (subsurface_surface);
    =
    for (GNode *G_PASTE(__n, __LINE__) = meta_get_first_subsurface_node ((surface)); \
     (subsurface = (G_PASTE (__n, __LINE__) ? G_PASTE (__n, __LINE__)->data : NULL)); \
     G_PASTE (__n, __LINE__) = meta_get_next_subsurface_sibling (G_PASTE (__n, __LINE__)))

In the new logic `subsurface` is still checked for NULL in the loop
condition. However, in the new loop init:

    ...
    meta_get_first_subsurface_node (MetaWaylandSurface *surface)
    ...

    n = g_node_first_child (surface->subsurface_branch_node);
    if (!G_NODE_IS_LEAF (n))
    ...

The above implementation performs a `G_NODE_IS_LEAF` call, which
performs a dereference on `n`, without first checking for NULLs.

This NULL dereference triggers the following gnome-shell crash:

    Core was generated by `/usr/bin/gnome-shell'.
    Program terminated with signal SIGSEGV, Segmentation fault.
    #0  meta_get_first_subsurface_node (surface=0x55d589623450) at ../src/wayland/meta-wayland-surface.h:399
    #1  pointer_can_grab_surface (pointer=0x7f6dc4012700, surface=0x55d589623450) at ../src/wayland/meta-wayland-pointer.c:1306
    #2  0x00007f6ddb94d509 in meta_wayland_pointer_can_grab_surface (pointer=<optimized out>, surface=surface@entry=0x55d589623450, serial=serial@entry=996) at ../src/wayland/meta-wayland-pointer.c:1321
    #3  0x00007f6ddb950d05 in meta_wayland_seat_get_grab_info (seat=seat@entry=0x55d586c24f20, surface=0x55d589623450, serial=996, require_pressed=require_pressed@entry=0, x=x@entry=0x0, y=y@entry=0x0)
        at ../src/wayland/meta-wayland-seat.c:467

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2655>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/wayland/meta-wayland-surface.h b/src/wayland/meta-wayland-surface.h
index 3ac1ab989..757de17d1 100644
--- a/src/wayland/meta-wayland-surface.h
+++ b/src/wayland/meta-wayland-surface.h
@@ -401,7 +401,9 @@ meta_get_first_subsurface_node (MetaWaylandSurface *surface)
   GNode *n;
 
   n = g_node_first_child (surface->subsurface_branch_node);
-  if (!G_NODE_IS_LEAF (n))
+  if (!n)
+    return NULL;
+  else if (!G_NODE_IS_LEAF (n))
     return n;
   else
     return meta_get_next_subsurface_sibling (n);