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 c4eb10e2f0, 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

lib/ssh.c

@@ -955,10 +955,10 @@ static CURLcode ssh_statemach_act(struct connectdata *conn, bool *block)
                                     sshc->sshagent_identity);
 
         if(rc < 0) {
-          if(rc != LIBSSH2_ERROR_EAGAIN)
+          if(rc != LIBSSH2_ERROR_EAGAIN) {
             /* tried and failed? go to next identity */
             sshc->sshagent_prev_identity = sshc->sshagent_identity;
-          else
+          }
           break;
         }
       }