Bug 1917218 - Fix crash in [@ nsMsgMdnGenerator::SendMdnMsg]. r=leftmostcat

Differential Revision: https://phabricator.services.mozilla.com/D221343

--HG--
extra : rebase_source : a9f7d186ba91ec041191d5d3f71cefcf47cb5339
extra : amend_source : cd7bd1b5c2e49c27453d5b64dddac770452eed3f

mailnews/extensions/mdn/nsMsgMdnGenerator.cpp

@@ -769,9 +769,7 @@ nsresult nsMsgMdnGenerator::SendMdnMsg() {
   rv = outgoingServerService->GetServerByIdentity(
       m_identity, getter_AddRefs(outgoingServer));
   NS_ENSURE_SUCCESS(rv, rv);
-  NS_ASSERTION(
-      outgoingServer,
-      "there should be an outgoing server configured (or set as default)");
+  NS_ENSURE_TRUE(outgoingServer, NS_ERROR_NULL_POINTER);
 
   nsCOMPtr<nsIMsgHeaderParser> parserService =
       do_GetService("@mozilla.org/messenger/headerparser;1", &rv);