paramiko 1.6.4 breaks if auth sock doesn't exist

[Martin Pool]

On one machine, for some mysterious reason, I have SSH_AUTH_SOCK
pointing to a socket that doesn't exist.  That gives me this traceback:

  File "/Users/mbp/bzr/Work/keepalive/bzrlib/transport/smart.py", line 1632, in _accept_bytes
    self._ensure_connection()
  File "/Users/mbp/bzr/Work/keepalive/bzrlib/transport/smart.py", line 1655, in _ensure_connection
    command=[executable, 'serve', '--inet', '--directory=/',
  File "/Users/mbp/bzr/Work/keepalive/bzrlib/transport/ssh.py", line 255, in connect_ssh
    t = self._connect(username, password, host, port)
  File "/Users/mbp/bzr/Work/keepalive/bzrlib/transport/ssh.py", line 243, in _connect
    _paramiko_auth(username, password, host, t)
  File "/Users/mbp/bzr/Work/keepalive/bzrlib/transport/ssh.py", line 382, in _paramiko_auth
    agent = paramiko.Agent()
  File "/Users/mbp/lib/python/paramiko/agent.py", line 61, in __init__
    conn.connect(os.environ['SSH_AUTH_SOCK'])
  File "<string>", line 1, in connect
error: (2, 'No such file or directory')

I suggest you should probably catch this (or all connection errors?),
and treat them as just not having an agent.

-- 
Martin