Fix transaction handling in invite form

src/pretix/control/views/auth.py

@@ -142,7 +142,9 @@ def invite(request, token):
 
     if request.method == 'POST':
         form = RegistrationForm(data=request.POST)
-        if form.is_valid():
+        with transaction.atomic():
+            valid = form.is_valid()
+            if valid:
                 user = User.objects.create_user(
                     form.cleaned_data['email'], form.cleaned_data['password'],
                     locale=request.LANGUAGE_CODE,
@@ -156,7 +158,6 @@ def invite(request, token):
                     settings.PRETIX_LONG_SESSIONS and form.cleaned_data.get('keep_logged_in', False)
                 )
 
-            with transaction.atomic():
                 inv.team.members.add(request.user)
                 inv.team.log_action(
                     'pretix.team.member.joined', data={