Merge pull request #805 from masamitsu-murase/add_field_write_barrier_to_guard_method_from_gc

Add field_write_barrier when an method is defined.

src/class.c

@@ -293,6 +293,9 @@ mrb_define_method_raw(mrb_state *mrb, struct RClass *c, mrb_sym mid, struct RPro
   if (!h) h = c->mt = kh_init(mt, mrb);
   k = kh_put(mt, h, mid);
   kh_value(h, k) = p;
+  if (p) {
+    mrb_field_write_barrier(mrb, (struct RBasic *)c, (struct RBasic *)p);
+  }
 }
 
 void
@@ -322,6 +325,9 @@ mrb_define_method_vm(mrb_state *mrb, struct RClass *c, mrb_sym name, mrb_value b
   k = kh_put(mt, h, name);
   p = mrb_proc_ptr(body);
   kh_value(h, k) = p;
+  if (p) {
+    mrb_field_write_barrier(mrb, (struct RBasic *)c, (struct RBasic *)p);
+  }
 }
 
 static mrb_value