Not checking for this would result in `clutter_actor_add_child`
failing, but StBin keeping a copy in `priv->child`. So later on,
`st_bin_remove` would never be called on it and this assertion
would fail and crash the whole shell:
```
static void
st_bin_destroy (ClutterActor *actor)
{
StBinPrivate *priv = st_bin_get_instance_private (ST_BIN (actor));
if (priv->child)
clutter_actor_destroy (priv->child);
g_assert (priv->child == NULL);
```
By disallowing spurious `st_bin_set_child` calls we now prevent StBin
from entering such a corrupt state and the above assertion won't fail
anymore.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1507>
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