MINOR: quic: complete traces/debug for handshake

Add more traces to follow CRYPTO data buffering in ncbuf. Offset for
quic_enc_level is now reported for event QUIC_EV_CONN_PRHPKTS. Also
ncb_advance() must never fail so a BUG_ON() statement is here to
guarantee it.

This was useful to track handshake failure reported too often. This is
related to github issue #1903.

This should be backported up to 2.6.
1 file changed