Fix warning about undefined @pipe in spec.

spec/async/io/protocol/line_spec.rb

@@ -30,7 +30,7 @@
 	let(:remote) {pipe.first}
 	subject {described_class.new(Async::IO::Stream.new(pipe.last, deferred: true), "\n")}
 
-	after(:each) {@pipe&.each(&:close)}
+	after(:each) {defined?(@pipe) && @pipe&.each(&:close)}
 
 	context "default line ending" do
 		subject {described_class.new(nil)}