multi: prevent nil panics in stop methods.

With this PR we might call the stop method even when the start
method of a subsystem did not successfully finish therefore we
need to make sure we guard the stop methods for potential panics
if some variables are not initialized in the contructors of the
subsystems.

chainntnfs/bitcoindnotify/bitcoind.go

@@ -151,7 +151,12 @@ func (b *BitcoindNotifier) Stop() error {
 
 		close(epochClient.epochChan)
 	}
-	b.txNotifier.TearDown()
+
+	// The txNotifier is only initialized in the start method therefore we
+	// need to make sure we don't access a nil pointer here.
+	if b.txNotifier != nil {
+		b.txNotifier.TearDown()
+	}
 
 	// Stop the mempool notifier.
 	b.memNotifier.TearDown()