Skip stock check when requested quantity is zero

This prevents an error being raised when available stock is negative

app/models/spree/line_item_decorator.rb

@@ -131,7 +131,9 @@ Spree::LineItem.class_eval do
 
   # MONKEYPATCH of Spree method
   # Enables scoping of variant to hub/shop, so we check stock against relevant overrides if they exist
+  # Also skips stock check if requested quantity is zero
   def sufficient_stock?
+    return true if quantity == 0 # This line added
     scoper.scope(variant) # This line added
     return true if Spree::Config[:allow_backorders]
     if new_record? || !order.completed?

spec/models/spree/line_item_spec.rb

@@ -76,6 +76,12 @@ module Spree
         expect(li.max_quantity).to eq 10
       end
 
+      it "caps at zero when stock is negative" do
+        v.update_attributes(on_hand: -2)
+        li.cap_quantity_at_stock!
+        expect(li.reload.quantity).to eq 0
+      end
+
       context "when a variant override is in place" do
         let!(:hub) { create(:distributor_enterprise) }
         let!(:vo) { create(:variant_override, hub: hub, variant: v, count_on_hand: 2) }
@@ -92,6 +98,16 @@ module Spree
           li.cap_quantity_at_stock!
           expect(li.quantity).to eq 2
         end
+
+        context "when count on hand is negative" do
+          before { vo.update_attributes(count_on_hand: -3) }
+
+          it "caps at zero" do
+            v.update_attributes(on_hand: -2)
+            li.cap_quantity_at_stock!
+            expect(li.reload.quantity).to eq 0
+          end
+        end
       end
     end