Set MRB_FIBER_TERMINATED flag on exception termination of a fiber.

src/vm.c

@@ -1902,6 +1902,7 @@ RETRY_TRY_BLOCK:
                 if (c->fib) {
                   mrb_write_barrier(mrb, (struct RBasic*)c->fib);
                 }
+                mrb->c->status = MRB_FIBER_TERMINATED;
                 mrb->c = c->prev;
                 c->prev = NULL;
                 goto L_RAISE;