add forgotten call to gr_initialize_field_data into cxx_example.C

src/example/cxx_example.C

@@ -77,6 +77,7 @@ int main(int argc, char *argv[])
 
   // Create struct for storing grackle field data
   grackle_field_data my_fields;
+  gr_initialize_field_data(&my_fields);
 
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.