caching_sha2_password/sha256_password login fails with passwords ≥ 20 characters

During RSA full authentication (non-TLS), the password is XOR-ed with the server's 20-byte scramble before encryption. When the scramble arrived via an AuthSwitchRequest it carried a trailing null, and the connector cycled the XOR over 21 bytes instead of 20, corrupting every password byte from index 20 on, so longer passwords were rejected.

Reported by 4UjwXc (GH #84).