rafthttp: return err if stopped before setting cancel in dial()

The original workflow may fail to cancel if stop() cancels the finished
request just before dial() assigning a new cancel. This commit checks
streamReader status before setting cancel to avoid this problem.

It is tested at travis for 300 times. go 1.5 always works well, while
go 1.4 fails to stop once.

rafthttp/stream.go

@@ -427,6 +427,12 @@ func (cr *streamReader) dial(t streamType) (io.ReadCloser, error) {
 	}
 
 	cr.mu.Lock()
+	select {
+	case <-cr.stopc:
+		cr.mu.Unlock()
+		return nil, fmt.Errorf("stream reader is stopped")
+	default:
+	}
 	cr.cancel = httputil.RequestCanceler(cr.tr, req)
 	cr.mu.Unlock()