BUG/MAJOR: quic: Wrong quic_max_available_room() returned value

BUG/MAJOR: quic: Wrong quic_max_available_room() returned value

Around limits for QUIC integer encoding, this functions could return
wrong values which lead to qc_build_frms() to prepare wrong CRYPTO (less chances)
or STREAM frames (more chances). qc_do_build_pkt() could build wrong packets
with bad CRYPTO/STREAM frames which could not be decoded by the peer.
In such a case ngtcp2 closes the connection with an ENCRYPTION_ERROR error
in a transport CONNECTION_CLOSE frame.