[Sync] Don't call start_callback_ if null in SMTP.

Still lost as to why it would be null, but this should stop crashes.

BUG=679657,673883

Review-Url: https://codereview.chromium.org/2622223003
Cr-Commit-Position: refs/heads/master@{#442978}

components/sync/model_impl/shared_model_type_processor.cc

@@ -102,17 +102,15 @@ void SharedModelTypeProcessor::ConnectIfReady() {
   if (start_error_) {
     error_handler_.Run(start_error_.value());
     start_error_.reset();
-    start_callback_.Reset();
-    return;
-  }
-
+  } else if (start_callback_) {
     auto activation_context = base::MakeUnique<ActivationContext>();
     activation_context->model_type_state = model_type_state_;
     activation_context->type_processor =
         base::MakeUnique<ModelTypeProcessorProxy>(
-          weak_ptr_factory_.GetWeakPtr(), base::ThreadTaskRunnerHandle::Get());
-
+            weak_ptr_factory_.GetWeakPtr(),
+            base::ThreadTaskRunnerHandle::Get());
     start_callback_.Run(std::move(activation_context));
+  }
   start_callback_.Reset();
 }