htlcswitch+itest: catch link quit signal when processing hodlQueue

This commit makes sure when processing resolutions, e.g, settling
invoices, when the link is already broken, the process would exit with
an error. This fixes the issue we found in the itest, where an
unexpected empty remote pending commitment was created although the
remote peer is already offline.

htlcswitch/link.go

@@ -1582,6 +1582,11 @@ loop:
 		select {
 		case item := <-l.hodlQueue.ChanOut():
 			htlcResolution = item.(invoices.HtlcResolution)
+
+		// No need to process it if the link is broken.
+		case <-l.cg.Done():
+			return ErrLinkShuttingDown
+
 		default:
 			break loop
 		}