veyron/services/wspr: returning from an error case to prevent panic

Change-Id: I4ff768841f26f84279e60684362a8b5f4b2a78ff
diff --git a/services/wspr/wsprd/lib/wspr.go b/services/wspr/wsprd/lib/wspr.go
index bb0256c..596ed12 100644
--- a/services/wspr/wsprd/lib/wspr.go
+++ b/services/wspr/wsprd/lib/wspr.go
@@ -461,6 +461,7 @@
 	stream := wsp.outstandingStreams[id].stream
 	if stream == nil {
 		w.sendError(fmt.Errorf("unknown stream"))
+		return
 	}
 
 	stream.Send(msg, w)