fix(extern_c_opr): let megbrain throw a exception when load extern c opr fails

GitOrigin-RevId: 16f5ceb8e3b0413de80812c3f3b3b75fa8205460

src/serialization/impl/extern_c_opr.cpp

@@ -518,6 +518,10 @@ cg::OperatorNodeBase* ExternCOprRunner::load(OprLoadContext& ctx,
     if (auto trans = iter->second.second) {
         desc = trans(desc);
     }
+
+    mgb_throw_if(nullptr == desc, MegBrainError,
+                 "loader create desc returns nullptr");
+
     return make_from_desc(dump_name, inputs, desc, config);
 }