Don't segfault if Warning.warn is undefined

	Check that there is a method entry for the method before passing
	it to rb_method_entry_arity.

	Fixes [Bug #18458]
	---
	 error.c                     | 3 ++-
	 test/ruby/test_exception.rb | 8 ++++++++
	 2 files changed, 10 insertions(+), 1 deletion(-)