Prevent div-by-zero error (#4967) (#4968)

- Div-by-zero could occur when calculating how many items can be built for a part
- Might result if (somehow) the BomItem has a quantity of zero

(cherry picked from commit d8965c6c2ba68a657b6cd489bd265bd63530f403)

Co-authored-by: Oliver <oliver.henry.walters@gmail.com>

InvenTree/part/models.py

@@ -1293,6 +1293,11 @@ class Part(InvenTreeBarcodeMixin, MetadataMixin, MPTTModel):
         )
 
         for item in queryset.all():
+
+            if item.quantity <= 0:
+                # Ignore zero-quantity items
+                continue
+
             # Iterate through each item in the queryset, work out the limiting quantity
             quantity = item.available_stock + item.substitute_stock