Use tor_mem_is_zero to check for broken node identity in node_is_a_configured_bridge()
In node_is_a_configured_bridge(), we use BUG(tor_digest_is_zero(...))
to check for a broken node_t whose digest is not set. But that's overkill: tor_digest_is_zero() uses the slow constant-time tor_memeq(), and this is a case where we will hit a BUG().