Avoid exception if email is wrong

Avoids:
```
AttributeError: 'NoneType' object has no attribute 'active'
  File "galaxy/web/framework/middleware/sentry.py", line 43, in __call__
    iterable = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/recursive.py", line 85, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/middleware/statsd.py", line 35, in __call__
    req = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/httpexceptions.py", line 640, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 136, in __call__
    return self.handle_request(environ, start_response)
  File "galaxy/web/framework/base.py", line 215, in handle_request
    body = method(trans, **kwargs)
  File "galaxy/webapps/galaxy/controllers/user.py", line 938, in activate
    if user.active is True:
```

lib/galaxy/webapps/galaxy/controllers/user.py

@@ -936,6 +936,9 @@ def activate(self, trans, **kwd):
         else:
             # Find the user
             user = trans.sa_session.query(trans.app.model.User).filter(trans.app.model.User.table.c.email == email).first()
+            if not user:
+                # Probably wrong email address
+                return trans.show_error_message("You are using an invalid activation link. Try to log in and we will send you a new activation email. <br><a href='%s'>Go to login page.</a>") % web.url_for(controller="root", action="index")
             # If the user is active already don't try to activate
             if user.active is True:
                 return trans.show_ok_message("Your account is already active. Nothing has changed. <br><a href='%s'>Go to login page.</a>") % web.url_for(controller='root', action='index')