Fix SSH state machine for ssh-agent authentication

[grembo]

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 c4eb10e,
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

[grembo]

How to reproduce:

1. ssh-add -D # clear ssh-agent
2. ssh-add ~/.ssh/wrongkey
3. ssh-add ~/.ssh/rightkey
4. curl sftp://myserver.example.com/file
5. Perceive authentication fail
6. ssh-add -D # clear ssh-agent
7. ssh-add ~/.ssh/rightkey
8. ssh-add ~/.ssh/wrongkey
9. curl sftp://myserver.example.com/file
10. Perceive authentication succeed

After applying the patch, 5. succeeds as expected.

[bagder]

Thanks!