irep is released when Fiber is terminated

src/gc.c

@@ -965,9 +965,11 @@ gc_gray_mark(mrb_state *mrb, mrb_gc *gc, struct RBasic *obj)
       size_t i;
       mrb_callinfo *ci;
 
-      if (!c) break;
+      if (!c || c->status == MRB_FIBER_TERMINATED) break;
+
       /* mark stack */
       i = c->stack - c->stbase;
+
       if (c->ci) {
         i += ci_nregs(c->ci);
       }