Don't panic when connect returns nil (fixes #1086)

It isn't clear to me why this would happen but let's guard the condition anyway.

src/core/link.go

@@ -273,7 +273,7 @@ func (l *links) add(u *url.URL, sintf string, linkType linkType) error {
 				}
 
 				conn, err := l.connect(state.ctx, u, info, options)
-				if err != nil {
+				if err != nil || conn == nil {
 					if linkType == linkTypePersistent {
 						// If the link is a persistent configured peering,
 						// store information about the connection error so