gc.c (mrb_field_write_barrier): value may be NULL
Especially when called from MRB_PROC_SET_TARGET_CLASS() from
f_instance_eval().
@@ -1400,6 +1400,7 @@ mrb_field_write_barrier(mrb_state *mrb, struct RBasic *obj, struct RBasic *value
{
mrb_gc *gc = &mrb->gc;
+ if (!value) return;
if (!is_black(obj)) return;
if (!is_white(value)) return;
if (is_red(value)) return;
