Avoid 0-length DATA if NGHTTP2_DATA_FLAG_NO_END_STREAM is set

lib/nghttp2_session.c

@@ -6906,6 +6906,14 @@ int nghttp2_session_pack_data(nghttp2_session *session, nghttp2_bufs *bufs,
 
   reschedule_stream(stream);
 
+  if (frame->hd.length == 0 && (data_flags & NGHTTP2_DATA_FLAG_EOF) &&
+      (data_flags & NGHTTP2_DATA_FLAG_NO_END_STREAM)) {
+    /* DATA payload length is 0, and DATA frame does not bear
+       END_STREAM.  In this case, there is no point to send 0 length
+       DATA frame. */
+    return NGHTTP2_ERR_CANCEL;
+  }
+
   return 0;
 }