[Bug #19242] Prohibit circular causes to be loaded

	---
	 error.c                     |  4 ++++
	 eval.c                      |  4 ++++
	 eval_error.c                | 11 +++++++++++
	 test/ruby/test_exception.rb | 12 ++++++++++++
	 4 files changed, 31 insertions(+)