net/http: update bundled x/net/http2

This updates the bundled copy of x/net/http2 to x/net git rev
3470a06c1357df53 for:

http2: fix typo in comment
https://golang.org/cl/44271

http2: fix nil dereference after Read completes with an error
https://golang.org/cl/44330

Fixes #20501

Change-Id: Ie6a5fb092f77044f504f146721dae86299e04011
Reviewed-on: https://go-review.googlesource.com/44331
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/h2_bundle.go

@@ -959,7 +959,7 @@ func http2configureTransport(t1 *Transport) (*http2Transport, error) {
 }
 
 // registerHTTPSProtocol calls Transport.RegisterProtocol but
-// convering panics into errors.
+// converting panics into errors.
 func http2registerHTTPSProtocol(t *Transport, rt RoundTripper) (err error) {
 	defer func() {
 		if e := recover(); e != nil {
@@ -3555,7 +3555,7 @@ func (p *http2pipe) Read(d []byte) (n int, err error) {
 		if p.breakErr != nil {
 			return 0, p.breakErr
 		}
-		if p.b.Len() > 0 {
+		if p.b != nil && p.b.Len() > 0 {
 			return p.b.Read(d)
 		}
 		if p.err != nil {
@@ -8811,7 +8811,7 @@ type http2PriorityWriteSchedulerConfig struct {
 }
 
 // NewPriorityWriteScheduler constructs a WriteScheduler that schedules
-// frames by following HTTP/2 priorities as described in RFC 7340 Section 5.3.
+// frames by following HTTP/2 priorities as described in RFC 7540 Section 5.3.
 // If cfg is nil, default options are used.
 func http2NewPriorityWriteScheduler(cfg *http2PriorityWriteSchedulerConfig) http2WriteScheduler {
 	if cfg == nil {