Fix #130268: Assertion triggered when animate property from Material panel

Fix an assertion that an embedded data-block has a zero 'real user'
count. Apparently it's possible for the shader node tree (embedded by
the material) to have a user count of 1.

Since that looks valid to me (only one user, namely the material itself)
I think it's fine to extend the assertion to that.

I did keep the assertion, to ensure that the embedded data-block is not
shared by multiple users. That shouldn't be possible, in any case.

Pull Request: https://projects.blender.org/blender/blender/pulls/130281

source/blender/animrig/intern/animdata.cc

@@ -74,7 +74,11 @@ Vector<ID *> find_related_ids(Main &bmain, ID &id)
 
     if (related_id->flag & ID_FLAG_EMBEDDED_DATA) {
       /* No matter the type of embedded ID, their owner can always be added to the related IDs. */
-      BLI_assert(ID_REAL_USERS(related_id) == 0);
+
+      /* User counting is irrelevant for the logic here, because embedded IDs cannot be shared.
+       * Embedded IDs do exist (sometimes) with a non-zero user count, hence the assertion that the
+       * user count is not greater than 1. */
+      BLI_assert(ID_REAL_USERS(related_id) <= 1);
       ID *owner_id = BKE_id_owner_get(related_id);
       /* Embedded IDs should always have an owner. */
       BLI_assert(owner_id != nullptr);