In rb_shape_rebuild_shape, we need to increase the capacity when
capacity == next_iv_index since the next ivar will be writing at index
next_iv_index.

This bug can be reproduced when assertions are turned on and you run the
following code:

    class Foo
      def initialize
        @A1 = 1
        @a2 = 1
        @A3 = 1
        @a4 = 1
        @A5 = 1
        @a6 = 1
        @a7 = 1
      end

      def add_ivars
        @a8 = 1
        @a9 = 1
      end
    end

    class Bar < Foo
    end

    foo = Foo.new
    foo.add_ivars
    bar = Bar.new
    GC.start
    bar.add_ivars
    bar.clone

You will get the following crash:

    Assertion Failed: object.c:301:rb_obj_copy_ivar:src_num_ivs <= shape_to_set_on_dest->capacity