BUG/MINOR: mux-quic: fix crash on qcs SD alloc failure

BUG/MINOR: mux-quic: fix crash on qcs SD alloc failure

Since the following commit, sedesc are created since QCS instantiation
in qcs_new().
  086e51017e7731ee9820b882fe6e8cc5f0dd5352
  BUG/MEDIUM: mux-quic: Create sedesc in same time of the QUIC stream

However, sedesc is initialized before other QCS mandatory fields. If
sedesc allocation fails, a crash would occur on qcs_free() invocation
for QCS early release. To fix this, delay sedesc allocation until
function end.

This bug was detected using -dMfail.

This should be backported up to 2.6.

(cherry picked from commit 3aded1d3752a12af9b8e48f445218230e6967a06)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>