Use a separate method to skip validation

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8091#note_20222666

lib/ci/api/builds.rb

@@ -41,7 +41,7 @@ module Ci
         put ":id" do
           authenticate_runner!
           build = Ci::Build.where(runner_id: current_runner.id).running.find(params[:id])
-          authenticate_build!(build, verify_token: false)
+          validate_build!(build)
 
           update_runner_info
 

lib/ci/api/helpers.rb

@@ -13,9 +13,14 @@ module Ci
         forbidden! unless current_runner
       end
 
-      def authenticate_build!(build, verify_token: true)
+      def authenticate_build!(build)
+        not_found! unless build
+        forbidden! if !build_token_valid?(build)
+        validate_build!(build)
+      end
+
+      def validate_build!(build)
         not_found! unless build
-        forbidden! if verify_token && !build_token_valid?(build)
         forbidden!('Project has been deleted!') unless build.project
         forbidden!('Build has been erased!') if build.erased?
       end