BUG/MAJOR: quic: Possible crash when processing 1-RTT during 0-RTT session

BUG/MAJOR: quic: Possible crash when processing 1-RTT during 0-RTT session

This bug was revealed by some C1 interop tests (heavy hanshake packet
corruption) when receiving 1-RTT packets with a key phase update.
This lead the packet to be decrypted with the next key phase secrets.
But this latter is initialized only after the handshake is complete.

In fact, 1-RTT must never be processed before the handshake is complete.
Relying on the "qc->mux_state == QC_MUX_NULL" condition to check the
handshake is complete is wrong during 0-RTT sessions when the mux
is initialized before the handshake is complete.

Must be backported to 2.7 and 2.6.