http2: avoid busy loop when ResponseHeaderTimeout is set

Don't keep reading from respHeaderRecv after the response headers
are received.

Fixes golang/go#49615.

Change-Id: Ib8126c954930011ac09b2cbc70bbbce76531b7db
Reviewed-on: https://go-review.googlesource.com/c/net/+/364574
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: neild

http2/transport.go

@@ -1313,6 +1313,7 @@ func (cs *clientStream) writeRequest(req *http.Request) (err error) {
 		case <-respHeaderTimer:
 			return errTimeout
 		case <-respHeaderRecv:
+			respHeaderRecv = nil
 			respHeaderTimer = nil // keep waiting for END_STREAM
 		case <-cs.abort:
 			return cs.abortErr