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

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

array.c

@@ -4845,6 +4845,7 @@ ary_append(VALUE x, VALUE y)
     if (n > 0) {
         rb_ary_splice(x, RARRAY_LEN(x), 0, RARRAY_CONST_PTR_TRANSIENT(y), n);
     }
+    RB_GC_GUARD(y);
     return x;
 }
 

test/ruby/test_array.rb

@@ -654,6 +654,12 @@ class TestArray < Test::Unit::TestCase
 
     assert_raise(TypeError) { [0].concat(:foo) }
     assert_raise(FrozenError) { [0].freeze.concat(:foo) }
+
+    a = @cls[nil]
+    def (x = Object.new).to_ary; Array.new(10) {nil} << :ok; end
+    EnvUtil.under_gc_stress {a.concat(x)}
+    GC.start
+    assert_equal(:ok, a.last)
   end
 
   def test_count