net/http2: update bundled http2

Update bundled http2 to git rev 76365a4 for https://golang.org/issue/18571

Fixes golang/go#13924

Change-Id: Ibb48cd6935b35d9965df70fb8761be5986d79ffc
Reviewed-on: https://go-review.googlesource.com/18591Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/h2_bundle.go

@@ -5038,7 +5038,7 @@ func (cc *http2ClientConn) streamByID(id uint32, andRemove bool) *http2clientStr
 	cc.mu.Lock()
 	defer cc.mu.Unlock()
 	cs := cc.streams[id]
-	if andRemove && cs != nil {
+	if andRemove && cs != nil && !cc.closed {
 		delete(cc.streams, id)
 		close(cs.done)
 	}