don't return null when dag file mismatch

[subtly]

The code below should either re-run the creation with force, delete the file and rerun, or inform the user to delete the file. Otherwise there is no way to determine what was wrong -- and a dag created by previous version of the software will fail.

if (file_size != found_size - ETHASH_DAG_MAGIC_NUM_SIZE) {
                fclose(f);
                ret = ETHASH_IO_MEMO_SIZE_MISMATCH;
                goto free_memo;
            }

[subtly]

Ah, I see that logic outside.