fix fiber GC problem

mrbgems/mruby-fiber/src/fiber.c

@@ -171,6 +171,8 @@ fiber_resume(mrb_state *mrb, mrb_value self)
     }
     c->cibase->argc = len;
     c->prev = mrb->c;
+    if (c->prev->fib) 
+      mrb_field_write_barrier(mrb, (struct RBasic*)c->fib, (struct RBasic*)c->prev->fib);
     c->status = MRB_FIBER_RUNNING;
     mrb->c = c;
 
@@ -179,6 +181,8 @@ fiber_resume(mrb_state *mrb, mrb_value self)
   }
   MARK_CONTEXT_MODIFY(c);
   c->prev = mrb->c;
+  if (c->prev->fib) 
+    mrb_field_write_barrier(mrb, (struct RBasic*)c->fib, (struct RBasic*)c->prev->fib);
   c->status = MRB_FIBER_RUNNING;
   mrb->c = c;
   return fiber_result(mrb, a, len);

src/gc.c

@@ -623,7 +623,7 @@ root_scan_phase(mrb_state *mrb)
   /* mark exception */
   mrb_gc_mark(mrb, (struct RBasic*)mrb->exc);
 
-  mark_context(mrb, mrb->c);
+  mark_context(mrb, mrb->root_c);
   /* mark irep pool */
   if (mrb->irep) {
     size_t len = mrb->irep_len;