BUILD: quic: fix dubious 0-byte overflow on qc_release_lost_pkts

BUILD: quic: fix dubious 0-byte overflow on qc_release_lost_pkts

With GCC 12.2.0 and O2 optimization activated, compiler reports the
following warning for qc_release_lost_pkts().

In function ‘quic_tx_packet_refdec’,
    inlined from ‘qc_release_lost_pkts.constprop’ at src/quic_conn.c:2056:3:
include/haproxy/atomic.h:320:41: error: ‘__atomic_sub_fetch_4’ writing 4 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
  320 | #define HA_ATOMIC_SUB_FETCH(val, i)     __atomic_sub_fetch(val, i, __ATOMIC_SEQ_CST)
      |                                         ^~~~~~~~~~~~~~~~~~
include/haproxy/quic_conn.h:499:14: note: in expansion of macro ‘HA_ATOMIC_SUB_FETCH’
  499 |         if (!HA_ATOMIC_SUB_FETCH(&pkt->refcnt, 1)) {
      |              ^~~~~~~~~~~~~~~~~~~

GCC thinks that quic_tx_packet_refdec() can be called with a NULL
argument from qc_release_lost_pkts() with <oldest_lost> as arg.

This warning is a false positive as <oldest_lost> cannot be NULL in
qc_release_lost_pkts() at this stage. This is due to the previous check
to ensure that <pkts> list is not empty.

This warning is silenced by using ALREADY_CHECKED() macro.

This should be backported up to 2.6.

This should fix github issue #1852.