Fix #105096: Material Utilities: Clean Material Slots fails to remove unused slots

The code was removing material slots while iterating through them which
is generally a bad idea, and it stopped working as the addon expected in
Blender 3.0+.

The slots are now removed by counting how many slots there are and then
removing that number of slots.

Pull Request: https://projects.blender.org/blender/blender-addons/pulls/105098

Author: Mysteryem

materials_utils/__init__.py

@@ -20,7 +20,7 @@
 bl_info = {
     "name": "Material Utilities",
     "author": "MichaleW, ChrisHinde",
-    "version": (2, 2, 1),
+    "version": (2, 2, 2),
     "blender": (3, 0, 0),
     "location": "View3D > Shift + Q key",
     "description": "Menu of material tools (assign, select..) in the 3D View",

materials_utils/functions.py

@@ -16,7 +16,7 @@ def mu_assign_material_slots(object, material_list):
     active_object = bpy.context.active_object
     bpy.context.view_layer.objects.active = object
 
-    for s in object.material_slots:
+    for _ in range(len(object.material_slots)):
         bpy.ops.object.material_slot_remove()
 
     # re-add them and assign material