protect returning irep (in proc) from GC

src/codegen.c

@@ -2906,6 +2906,7 @@ mrb_generate_code(mrb_state *mrb, parser_state *p)
     // prepare irep
     codegen(scope, p->tree, NOVAL);
     proc = mrb_proc_new(mrb, scope->irep);
+    mrb_irep_decref(mrb, scope->irep);
     mrb_pool_close(scope->mpool);
     return proc;
   }