VOL-1944: Print backend name assigned only if non-nil or was assigned.

Change-Id: I3eee44fecd06cf7806778d19229a81d011ae60d9
diff --git a/afrouter/afrouter/cluster.go b/afrouter/afrouter/cluster.go
index 7fc581b..bb95183 100644
--- a/afrouter/afrouter/cluster.go
+++ b/afrouter/afrouter/cluster.go
@@ -157,7 +157,6 @@
 	if err := src.RecvMsg(f); err != nil {
 		return nil, err
 	}
-	log.Debugf("Assigned backend %s", f.backend.name)
 	// Check that the backend was routable and actually has connections open.
 	// If it doesn't then return a nil backend to indicate this
 	if f.backend == nil {
@@ -169,5 +168,6 @@
 		log.Error(err)
 		return f.backend, err
 	}
+	log.Debugf("Assigned backend %s", f.backend.name)
 	return f.backend, nil
 }