Fix wait time computation with auth_user

Fix wait time computation with auth_user

When using auth_user, the transition to the CL_WAITING_LOGIN state
would not initialize the client->wait_start field.  This would either
lead to garbage values being recorded, or under assertions enabled it
would crash in activate_client().

(test_auth_user was actually reproducing this problem, but a crash
requires assertions enabled and new memory being all zero, so it was
difficult to catch it.)

Author: @pinaraf

see #393