Raise exception if dividing by 0

While this should never happen it is possible to call this method
and subsequently try to save a NaN value if the line_item has
already been fully cancelled.

core/app/models/spree/unit_cancel.rb

@@ -41,7 +41,10 @@ def compute_amount(line_item)
   private
 
   def weighted_line_item_amount(line_item)
-    line_item.total_before_tax / quantity_of_line_item(line_item)
+    quantity_of_line_item = quantity_of_line_item(line_item)
+    raise ZeroDivisionError, "Line Item does not have any inventory units available to cancel" if quantity_of_line_item.zero?
+
+    line_item.total_before_tax / quantity_of_line_item
   end
 
   def quantity_of_line_item(line_item)

core/spec/models/spree/unit_cancel_spec.rb

@@ -48,6 +48,11 @@
       it "divides the line item total by the uncanceled units size" do
         expect(subject).to eq(-10.0)
       end
+
+      it "raises an error if dividing by 0" do
+        inventory_unit.cancel!
+        expect { subject }.to raise_error ZeroDivisionError, "Line Item does not have any inventory units available to cancel"
+      end
     end
 
     context "it is called with a line item that doesnt belong to the inventory unit" do