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.

(cherry picked from commit ffa926ead3f17cd8da773daa137184619bfc1a0a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 46a8238ca06f28a057658c2dade22ac894c571cf)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit cc3c9a786f8eeba582342838014c2c2e29c3b4da)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>