Merge pull request #2867 from kou/proc-fix-gc-crash

Fix a crash bug when GC is ran while creating a proc with env

src/proc.c

@@ -93,6 +93,7 @@ mrb_proc_new_cfunc_with_env(mrb_state *mrb, mrb_func_t func, mrb_int argc, const
   int i;
 
   p->env = e = env_new(mrb, argc);
+  mrb_field_write_barrier(mrb, (struct RBasic *)p, (struct RBasic *)p->env);
   MRB_ENV_UNSHARE_STACK(e);
   e->stack = (mrb_value*)mrb_malloc(mrb, sizeof(mrb_value) * argc);
   if (argv) {