[Bug #18329] Fix crash when calling non-existent super method

	The cme is NULL when a method does not exist, so check it before
	accessing the callcache.
	---
	 test/ruby/test_super.rb | 31 +++++++++++++++++++++++++++++++
	 vm_insnhelper.c         |  3 ++-
	 2 files changed, 33 insertions(+), 1 deletion(-)