Do not generate Change on unsaved object

Fixes #1005
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/subproject.py

@@ -854,6 +854,7 @@ class SubProject(models.Model, PercentMixin, URLMixin, PathMixin):
             translation,
             str(error)
         )
+        if self.id:
             Change.objects.create(
                 subproject=self,
                 translation=translation,