TST: Add test for erroneous void pointee assignment

test/test_ccaosdb.cpp

@@ -278,6 +278,9 @@ TEST_F(test_ccaosdb, test_entity_with_parent_and_property) {
   char out[255] = {"b"}; // NOLINT
 
   std::cout << "Comparing ..." << std::endl;
+  // cannot assign an already assigned property
+  return_code = caosdb_entity_entity_get_property(&entity, &input_property, 0);
+  EXPECT_EQ(return_code, caosdb::StatusCode::EXTERN_C_ASSIGNMENT_ERROR);
   caosdb_entity_property output_property;
   return_code = caosdb_entity_entity_get_property(&entity, &output_property, 0);
   std::cout << "Got output property." << std::endl;