Garbage collector crash

How it was found

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