Prohibit calling undefined allocator [Bug #16297]

	---
	 object.c                | 31 +++++++++++++++++++++++++++++--
	 test/ruby/test_class.rb |  8 ++++++++
	 2 files changed, 37 insertions(+), 2 deletions(-)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67896 b2dd03c8-39d4-4d8f-98ff-823fe69b080e