if (n_chan_id) in circuit_build_failed() can't fail

In circuit_build_failed() we have

    const char *n_chan_id = circ->cpath->extend_info->identity_digest;
[...]
    if (n_chan_id && !already_marked) {

But extend_info->identity_digest is

  char identity_digest[DIGEST_LEN];

So n_chan_id is never going to be NULL.

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