[ruby/pathname] Fix segfault of Pathname#split

Fix segmentation fault of Pathname#split when File.split returns
non array value [Bug #17755]

https://github.com/ruby/pathname/commit/e29b49e3b1
https://github.com/ruby/pathname/commit/1db7479a74

ext/pathname/pathname.c

@@ -834,7 +834,7 @@ path_split(VALUE self)
     VALUE str = get_strpath(self);
     VALUE ary, dirname, basename;
     ary = rb_funcall(rb_cFile, id_split, 1, str);
-    ary = rb_check_array_type(ary);
+    Check_Type(ary, T_ARRAY);
     dirname = rb_ary_entry(ary, 0);
     basename = rb_ary_entry(ary, 1);
     dirname = rb_class_new_instance(1, &dirname, rb_obj_class(self));

test/pathname/test_pathname.rb

@@ -1067,6 +1067,21 @@ class TestPathname < Test::Unit::TestCase
 
   def test_split
     assert_equal([Pathname("dirname"), Pathname("basename")], Pathname("dirname/basename").split)
+
+    assert_separately([], <<-'end;')
+      require 'pathname'
+
+      mod = Module.new do
+        def split(_arg)
+        end
+      end
+
+      File.singleton_class.prepend(mod)
+
+      assert_raise(TypeError) do
+        Pathname('/').split
+      end
+    end;
   end
 
   def test_blockdev?