peer: panic early on queued nil messages

peer/peer.go

@@ -1688,6 +1688,14 @@ cleanup:
 //
 // This function is safe for concurrent access.
 func (p *Peer) QueueMessage(msg wire.Message, doneChan chan<- struct{}) {
+	// Panic early on nil messages.  This provides a more useful stack
+	// trace to callers than hitting the panic in a long-lived peer
+	// goroutine that contains no information about what caller queued the
+	// nil message.
+	if msg == nil {
+		panic("peer: nil message")
+	}
+
 	// Avoid risk of deadlock if goroutine already exited.  The goroutine
 	// we will be sending to hangs around until it knows for a fact that
 	// it is marked as disconnected and *then* it drains the channels.