Guard array when appending

This prevents early collection of the array.  The GC doesn't see the
array on the stack when Ruby is compiled with optimizations enabled

Thanks @jhaberman for the test case

[ruby-core:105099] [Bug #18140]

test/-ext-/array/test_to_ary_concat.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: false
+require 'test/unit'
+require '-test-/array/to_ary_concat'
+
+class TestConcatStress < Test::Unit::TestCase
+  def setup
+    @stress_level = GC.stress
+    GC.stress = true
+  end
+
+  def teardown
+    GC.stress = @stress_level
+  end
+
+  def test_concat
+    arr = [nil]
+    bar = Bug::Bar.new
+    arr.concat(bar)
+  end
+end