title: django-allauth social account connect to existing account on login
body: <p>I have a custom user model and I am using django-allauth for social registration and login. 
  I am trying to connect existing user to new social account when a user login using a social account who already has registered using email. I found this <a href="https://github.com/pennersr/django-allauth/issues/418#issuecomment-38391830" rel="noreferrer">link</a>.</p>
  
  <pre><code>def pre_social_login(self, request, sociallogin):
      user = sociallogin.account.user
      if user.id:
          return
      try:
          customer = Customer.objects.get(email=user.email)
      except Customer.DoesNotExist:
          pass
      else:
          perform_login(request, customer, 'none')
  </code></pre>
  
  <p>But I'm getting an error when I try to login through social account.</p>
  
  <pre><code>RelatedObjectDoesNotExist at /accounts/facebook/login/callback/
  SocialAccount has no user.
  </code></pre>
  
  <p>Any help will be appreciated.</p>
  
  <p>Also I am aware of the security issue in this. But I still want to try this.</p>