Fix assertion failure
Fix assertion failure

Fix assertion failure which occurs when CWND is filled up with
Handshake probe packet and Short packet in the first call of
ngtcp2_conn_writev_stream with NGTCP2_WRITE_STREAM_FLAG_MORE flag set.
The next ngtcp2_conn_writev_stream gets assertion failure.

ngtcp2/ngtcp2,ngtcp2/ngtcp2,ngtcp2/ngtcp2