ssh: Handle successful SSH_USERAUTH_NONE

According to the documentation for libssh2_userauth_list(), a NULL
return value is not necessarily an error. You must call
libssh2_userauth_authenticated() to determine if the SSH_USERAUTH_NONE
request was successful.

This fixes a segv when using sftp on a server that allows logins with an
empty password. When NULL was interpreted as an error, it would
free the session but not flag an error since the libssh2 errno would be
clear. This resulted in dereferencing a NULL session pointer.

Signed-off-by: Tyler Hall <tylerwhall@gmail.com>