Stop reading past the end of `ivptr` array

	If you have code like this:

	```ruby
	class A
  def initialize
    @a = nil
    @b = nil
    @c = nil
    @d = nil
    @e = nil
  end
	end

	x = A.new
	y = x.clone
	100.times { |z| x.instance_variable_set(:"@foo#{z}", nil) }
	puts y.inspect
	```

	`x` and `y` will share `iv_index_tbl` hashes.  However, the size of the
	hash will grow larger than the number if entries in `ivptr` in `y`.
	Before this commit, `rb_ivar_count` would use the size of the hash to
	determine how far to read in to the array, but this means that it could
	read past the end of the array and cause the program to segv

	[ruby-core:78403]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@57214 b2dd03c8-39d4-4d8f-98ff-823fe69b080e