Null pointer dereference in ary_concat

The following file causes a segmentation fault in mruby, which also causes a segmentation fault in mruby-engine. I've minimized this file down to the bare bones what crashes it, then renamed variables and tidied so you can see what is needed and what isn't.

a=[0]
b=nil
a.each do |a|
  a = *case a
    when b
    redo
  end
end