merge revision(s) c9423b01,0b1e2639,78ee2c24: [Backport #15968]

	marshal.c: check instance variable count

	* marshal.c (w_obj_each): ensure that no instance variable was
	  added while dumping other instance variables.  [Bug #15968]

	Hoisted out w_ivar_each


	marshal.c: check instance variable count

	* marshal.c (w_ivar_each): ensure that no instance variable was
	  removed while dumping other instance variables.  [Bug #15968]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67832 b2dd03c8-39d4-4d8f-98ff-823fe69b080e