BUG/MINOR: mux-quic: trace with non initialized qcc

BUG/MINOR: mux-quic: trace with non initialized qcc

This issue leads to crashes when the QUIC mux traces are enabled and could be
reproduced with -dMfail. When the qcc allocation fails (qcc_init()) haproxy
crashes into qmux_dump_qcc_info() because ->conn qcc member is initialized:

Program terminated with signal SIGSEGV, Segmentation fault.
    at src/qmux_trace.c:146
146             const struct quic_conn *qc = qcc->conn->handle.qc;
[Current thread is 1 (LWP 1448960)]
(gdb) p qcc
$1 = (const struct qcc *) 0x7f9c63719fa0
(gdb) p qcc->conn
$2 = (struct connection *) 0x155550508
(gdb)

This patch simply fixes the TRACE() call concerned to avoid <qcc> object
dereferencing when it is NULL.

Must be backported as far as 3.0.