merge revision(s) 40791,40806: [Backport #8424]

	* eval_intern.h (TH_PUSH_TAG): ensure jmpbuf to be accessible before
	  pushing tag to get rid of unaccessible tag by stack overflow.

	* vm_core.h (rb_vm_tag): move jmpbuf between tag and prev so ensure to
	  be accessible.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@41054 b2dd03c8-39d4-4d8f-98ff-823fe69b080e