fail gracefully in the absence of an oauth session

closes FOO-1462
flag=none

TEST PLAN:
  1) hit the deny endpoint with no session info
  2) you should get a 400, not a 500

Change-Id: I0cf286e283bdad12c9bd65a6a53b8bd052a3d3ef
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/256669
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

app/controllers/oauth2_provider_controller.rb

@@ -92,6 +92,7 @@ class Oauth2ProviderController < ApplicationController
   end
 
   def deny
+    return render plain: t("Invalid or missing session for oauth"), status: 400 unless session[:oauth2]
     params = { error: "access_denied" }
     params[:state] = session[:oauth2][:state] if session[:oauth2].key? :state
     redirect_to Canvas::Oauth::Provider.final_redirect(self, params)

spec/controllers/oauth2_provider_controller_spec.rb

@@ -724,6 +724,11 @@ describe Oauth2ProviderController do
       expect(response).to be_redirect
       expect(response.location).not_to match(/state=/)
     end
+
+    it "doesn't error on an empty session" do
+      get 'deny', session: {}
+      expect(response).to be_bad_request
+    end
   end
 
   describe 'DELETE token' do