BUG/MINOR: quic/ssl: crash in ClientHello callback ssl traces

BUG/MINOR: quic/ssl: crash in ClientHello callback ssl traces

Such crashes may occur for QUIC frontends only when the SSL traces are enabled.

ssl_sock_switchctx_cbk() ClientHello callback may be called without any connection
initialize (<conn>) for QUIC connections leading to crashes when passing
conn->err_code to TRACE_ERROR().

Modify the TRACE_ERROR() statement to pass this parameter only when <conn> is
initialized.

Must be backported as far as 3.2.