Garbage collector crash

This github [issue](https://github.com/mruby/mruby/issues/3063] seems to have been reintroduced. f = Fiber.new do m = Fiber.current Fiber.yield Proc.new {} end f = f.resume GC.start It causes mruby to abort due to a failed assertion. $ mruby poc mruby: /home/user/repos/mruby/src/gc.c:698: mrb_gc_mark: Assertion `(obj)->tt != MRB_TT_FREE' failed. Aborted The issue was reintroduced in ecee8c51b0ad8cddd9e422a3e5105f902d7e2781 and is still present in 051e40c0493f2de332f5439e3230c9fe6958bf1a. The issue is fixed by reverting ecee8c51b0ad8cddd9e422a3e5105f902d7e2781. Thank you, Dinko Galetic Denis Kasak