MINOR: quic: Possible crash when inspecting the xprt context

MINOR: quic: Possible crash when inspecting the xprt context

haproxy may crash when running this statement in qc_lstnr_pkt_rcv():
	conn_ctx = qc->conn->xprt_ctx;
because qc->conn may not be initialized. With this patch we ensure
qc->conn is correctly initialized before accessing its ->xprt_ctx
members. We zero the xrpt_ctx structure (ssl_conn_ctx struct), then
initialize its ->conn member with HA_ATOMIC_STORE. Then, ->conn and
->conn->xptr_ctx members of quic_conn struct can be accessed with HA_ATOMIC_LOAD()