Fix make single user crash

How to reproduce the crash:
* Factory startup
* 'u'key (make single user)

It comes with a simple unittest to reproduce the original issue.

source/blender/editors/object/object_relations.c

@@ -1632,7 +1632,6 @@ void OBJECT_OT_make_links_data(wmOperatorType *ot)
 
 static Object *single_object_users_object(Main *bmain, Scene *scene, Object *ob, const bool copy_groups)
 {
-	if (!ID_IS_LINKED(ob) && ob->id.us > 1) {
 	/* base gets copy of object */
 	Object *obn = ID_NEW_SET(ob, BKE_object_copy(bmain, ob));
 
@@ -1657,8 +1656,6 @@ static Object *single_object_users_object(Main *bmain, Scene *scene, Object *ob,
 
 	id_us_min(&ob->id);
 	return obn;
-	}
-	return NULL;
 }
 
 static void libblock_relink_scene_collection(SceneCollection *sc)
@@ -1678,9 +1675,11 @@ static void single_object_users_scene_collection(Main *bmain, Scene *scene, Scen
 		Object *ob = link->data;
 		/* an object may be in more than one collection */
 		if ((ob->id.newid == NULL) && ((ob->flag & flag) == flag)) {
+			if (!ID_IS_LINKED(ob) && ob->id.us > 1) {
 				link->data = single_object_users_object(bmain, scene, link->data, copy_groups);
 			}
 		}
+	}
 
 	for (SceneCollection *nsc = sc->scene_collections.first; nsc; nsc = nsc->next) {
 		single_object_users_scene_collection(bmain, scene, nsc, flag, copy_groups);

tests/python/view_layer/CMakeLists.txt

@@ -108,6 +108,7 @@ VIEW_LAYER_TEST(object_link_a)
 VIEW_LAYER_TEST(object_link_b)
 VIEW_LAYER_TEST(object_link_c)
 VIEW_LAYER_TEST(operator_context)
+VIEW_LAYER_TEST(make_single_user)
 VIEW_LAYER_TEST(move_above_below_scene_collection_a)
 VIEW_LAYER_TEST(move_above_below_scene_collection_b)
 VIEW_LAYER_TEST(move_above_below_scene_collection_c)

tests/python/view_layer/test_make_single_user.py

@@ -0,0 +1,54 @@
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+import os
+import sys
+
+from view_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(ViewLayerTesting):
+    def test_make_single_user(self):
+        """
+        Really basic test, just to check for crashes on basic files.
+        """
+        import bpy
+        scene = bpy.context.scene
+        master_collection = scene.master_collection
+        view_layer = bpy.context.view_layer
+        ob = bpy.context.object
+
+        # clean up the scene a bit
+        for o in (o for o in view_layer.objects if o != ob):
+            view_layer.collections[0].collection.objects.unlink(o)
+
+        for v in (v for v in scene.view_layers if v != view_layer):
+            scene.view_layers.remove(v)
+
+        while master_collection.collections:
+            master_collection.collections.remove(
+                    master_collection.collections[0])
+
+        view_layer.collections.link(master_collection)
+        ob.select_set('SELECT')
+
+        # update depsgraph
+        scene.update()
+
+        # test itself
+        bpy.ops.object.make_single_user(object=True)
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    UnitTesting._extra_arguments = setup_extra_arguments(__file__)
+    unittest.main()