transport: Fix a data race when headers are received while the stream is being closed (#1814)

transport/http2_client.go

@@ -1116,13 +1116,14 @@ func (t *http2Client) operateHeaders(frame *http2.MetaHeadersFrame) {
 	}()
 
 	s.mu.Lock()
+	if !s.headerDone {
+		// Headers frame is not actually a trailers-only frame.
 		if !endStream {
 			s.recvCompress = state.encoding
-	}
-	if !s.headerDone {
-		if !endStream && len(state.mdata) > 0 {
+			if len(state.mdata) > 0 {
 				s.header = state.mdata
 			}
+		}
 		close(s.headerChan)
 		s.headerDone = true
 		isHeader = true