Fix ssh_handle_key_exchange() timeout

See #311 (closed) for background. But in some case, it's possible to trigger the code in ssh_handle_key_exchange() to move session state directly to SSH_SESSION_STATE_AUTHENTICATED. The exit condition for this function is SSH_SESSION_STATE_AUTHENTICATING though, so when it happens, ssh_handle_key_exchange() will time out eventually.

The fix is to check DH_STATE_FINISHED instead, which should be the case for both AUTHENTICATED and AUTHENTICATION. Suggested by Eshan Kelkar. Tested with the problematic client (trilead-ssh2) and made sure the bad condition happened (and not cause timeout)

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code