curl: 12: SASL DIGEST-MD5 does not validate the server's `rspauth` proof

Summary curl never verifies the rspauth value the server sends during SASL DIGEST-MD5 authentication. In Curlsaslcontinue lib/curlsasl.c, the SASLDIGESTMD5RESP handler unconditionally sends an empty acknowledgment without reading or checking the server's continuation data. Authentication then...