fix: do not save the domain if not necessary.

it will also avoid the trigger of two post_save signals for the domain,
which can lead to unexpected issues.

see #1380

modoboa/relaydomains/forms.py

@@ -37,6 +37,7 @@ class RelayDomainFormGeneral(tr_forms.TransportForm):
         instance.pattern = domain.name
         instance.service = "relay"
         instance.save()
-        domain.transport = instance
-        domain.save(update_fields=["transport"])
+        if not domain.transport:
+            domain.transport = instance
+            domain.save(update_fields=["transport"])
         return instance