Assertion failure in mrb_gc_mark

[clayton-shopify]

The following input demonstrates a crash, when supplied to mirb:

def a
x
ensure
e
end

k = Numeric.new  

def z;e;end

def e
h {}
end

def h
a
ensure
x
end

Symbol.all_symbols  []

t = Fiber.new do
u = k.to_s 
end


z.instance_exec  {}
z.instance_exec  {}


t.resume

z[0]

GC.start

Result:

Assertion failed: ((obj)->tt != MRB_TT_FREE), function mrb_gc_mark, file /Users/clayton/git/mruby/src/gc.c, line 714.
Abort trap: 6

This issue was reported by https://hackerone.com/ahihi