Running cube example in readme can crash on some files

[braingram]

The example contains:

    // This reads the top-level core/asdf-1.0.0 schema
    err = asdf_get_meta(file, "/", &meta);
    if (err == ASDF_VALUE_OK) {
        if (meta->history.entries[0]) {
            // This is a NULL-terminated array of asdf_history_entry_t*
            printf("first history entry: %s\n", meta->history.entries[0]->description);
        }
    }

when run with a file without a history entry the entries are indexed out of bounds and the program crashes (segfaults).

This example should be updated to check if an entry exists and produce an informative error if there is none.

[braingram]

It would also be helpful if the expected input file was available and mentioned alongside the example.