vendor: update buildkit to 539be170

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/golang.org/x/net/http2/pipe.go

@@ -30,6 +30,17 @@ type pipeBuffer interface {
 	io.Reader
 }
 
+// setBuffer initializes the pipe buffer.
+// It has no effect if the pipe is already closed.
+func (p *pipe) setBuffer(b pipeBuffer) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.err != nil || p.breakErr != nil {
+		return
+	}
+	p.b = b
+}
+
 func (p *pipe) Len() int {
 	p.mu.Lock()
 	defer p.mu.Unlock()