Denial of service due to invalid memory access in mrb_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.

case ""
  when 0
end
x *case
  when true
    * = 0
end