Give tor_cert_get_id_digests() fail-fast behavior

Right now we can take the digests only of an RSA key, and only expect to
take the digests of an RSA key.  The old tor_cert_get_id_digests() would
return a good set of digests for an RSA key, and an all-zero one for a
non-RSA key.  This behavior is too error-prone: it carries the risk that
we will someday check two non-RSA keys for equality and conclude that
they must be equal because they both have the same (zero) "digest".

Instead, let's have tor_cert_get_id_digests() return NULL for keys we
can't handle, and make its callers explicitly test for NULL.

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