BUG/MINOR: connection: Missing QUIC initialization

BUG/MINOR: connection: Missing QUIC initialization

The QUIC connection struct connection member was not initialized. This may
make randomly haproxy handle TLS connections as QUIC ones only when QUIC support
is enabled leading to such OpenSSL errors (captured from a reg test output, TLS
Client-Hello callback failed):

    OpenSSL error[0x10000085] OPENSSL_internal: CONNECTION_REJECTED
    OpenSSL error[0x10000410] OPENSSL_internal: SSLV3_ALERT_HANDSHAKE_FAILURE
    OpenSSL error[0x1000009a] OPENSSL_internal: HANDSHAKE_FAILURE_ON_CLIENT_HELLO

This patch should fix #1168 github issue.