merge revision(s) 5d33f787,8b162ce9: [Backport #14834]

	fix tracepoint + backtrace SEGV

	PC modification in gc_event_hook_body was careless.  There are (so
	to say) abnormal iseqs stored in the cfp.  We have to check sanity
	before we touch the PC.

	This has not been fixed because there was no way to (ab)use the
	setup from pure-Ruby.  However by using our official C APIs it is
	possible to touch such frame(s), resulting in SEGV.

	Fixes [Bug #14834].

	Fix assertion failure when VM_CHECK_MODE

	Some VM frames (dummy and top pushed by `rb_vm_call_cfunc`) has
	iseq but has no pc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67745 b2dd03c8-39d4-4d8f-98ff-823fe69b080e