SSH: Fix state machine for ssh-agent authentication

In case an identity didn't match[0], the state machine would fail in
state SSH_AUTH_AGENT instead of progressing to the next identity in
ssh-agent. As a result, ssh-agent authentication only worked if the
identity required happened to be the first added to ssh-agent.

This was introduced as part of commit c4eb10e2, which
stated that the "else" statement was required to prevent getting stuck
in state SSH_AUTH_AGENT. Given the state machine's logic and libssh2's
interface I couldn't see how this could happen or reproduce it and I
also couldn't find a more detailed description of the problem which
would explain a test case to reproduce the problem this was supposed to
fix.

[0] libssh2_agent_userauth returning LIBSSH2_ERROR_AUTHENTICATION_FAILED

Closes #2248