Mark mrb->root_c->fib in root scan phase if exists.

This is a pull request conflicting with #1865.
This solves same problem modifying **gc.c* instead.
After rethinking this seems more smarter than #1865.
The failing test is at https://travis-ci.org/take-cheeze/mruby/builds/20929102 .(It should show "unexpected type tag: 1" though it got SEGV instead)