[xcassets] Some protection against potentially trying to remove already-non-existing references

(this was an attempt to get rid of (null) references in the generated Xcode project; it finally wasn't the cause, but keeping the test for safety)

lib/cocoapods/installer/user_project_integrator/target_integrator.rb

@@ -176,10 +176,12 @@ module Pod
           # Remove the resources no longer in a target
           refs_to_remove.each do |file_ref|
             native_targets.each do |user_target|
+              if user_target.resources_build_phase.include?(file_ref)
                 user_target.resources_build_phase.remove_file_reference(file_ref)
                 dirty = true
               end
             end
+          end
 
           dirty
         end
@@ -191,10 +193,12 @@ module Pod
         def remove_pods_resources
           TargetIntegrator.each_pods_resources(user_project) do |file_ref|
             native_targets.each do |user_target|
+              if user_target.resources_build_phase.include?(file_ref)
                 user_target.resources_build_phase.remove_file_reference(file_ref)
               end
             end
           end
+        end
 
         # Iterate over each file in the 'Pods/Resources' group of the given `project`
         #