small fixes for robustness and readability

src/sysc/core_complex.cpp

@@ -290,6 +290,7 @@ void core_complex::before_end_of_elaboration() {
     cpu->set_mhartid(mhartid.get_value());
 
     vm = create_cpu(cpu.get(), backend.get_value(), gdb_server_port.get_value());
+    sc_assert(vm!=nullptr);
 #ifdef WITH_SCV
     vm->setDisassEnabled(enable_disass.get_value() || m_db != nullptr);
 #else