http2: prevent uninitialized pipe from being written

For golang/go#65927

Change-Id: I6f48706156384e026968cf9a6d9e0ec76b46fabf
Reviewed-on: https://go-review.googlesource.com/c/net/+/566675
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

http2/pipe.go

@@ -77,7 +77,10 @@ func (p *pipe) Read(d []byte) (n int, err error) {
 	}
 }
 
-var errClosedPipeWrite = errors.New("write on closed buffer")
+var (
+	errClosedPipeWrite        = errors.New("write on closed buffer")
+	errUninitializedPipeWrite = errors.New("write on uninitialized buffer")
+)
 
 // Write copies bytes from p into the buffer and wakes a reader.
 // It is an error to write more data than the buffer can hold.
@@ -91,6 +94,12 @@ func (p *pipe) Write(d []byte) (n int, err error) {
 	if p.err != nil || p.breakErr != nil {
 		return 0, errClosedPipeWrite
 	}
+	// pipe.setBuffer is never invoked, leaving the buffer uninitialized.
+	// We shouldn't try to write to an uninitialized pipe,
+	// but returning an error is better than panicking.
+	if p.b == nil {
+		return 0, errUninitializedPipeWrite
+	}
 	return p.b.Write(d)
 }