BUILD: checks: shut up yet another stupid gcc warning

BUILD: checks: shut up yet another stupid gcc warning

gcc has always had hallucinations regarding value ranges, and this one
is interesting, and affects branches 4.7 to 11.3 at least. When building
without threads, the randomly picked new_tid that is reduced to a multiply
by 1 shifted right 32 bits, hence a constant output of 0 shows this
warning:

  src/check.c: In function 'process_chk_conn':
  src/check.c:1150:32: warning: array subscript [-1, 0] is outside array bounds of 'struct thread_ctx[1]' [-Warray-bounds]
  In file included from include/haproxy/thread.h:28,
                   from include/haproxy/list.h:26,
                   from include/haproxy/action.h:28,
                   from src/check.c:31:

or this one when trying to force the test to see that it cannot be zero(!):

  src/check.c: In function 'process_chk_conn':
  src/check.c:1150:54: warning: array subscript [0, 0] is outside array bounds of 'struct thread_ctx[1]' [-Warray-bounds]
   1150 |         uint t2_act  = _HA_ATOMIC_LOAD(&ha_thread_ctx[thr2].active_checks);
        |                                         ~~~~~~~~~~~~~^~~~~~
  include/haproxy/atomic.h:66:40: note: in definition of macro 'HA_ATOMIC_LOAD'
     66 | #define HA_ATOMIC_LOAD(val)          *(val)
        |                                        ^~~
  src/check.c:1150:24: note: in expansion of macro '_HA_ATOMIC_LOAD'
   1150 |         uint t2_act  = _HA_ATOMIC_LOAD(&ha_thread_ctx[thr2].active_checks);
        |                        ^~~~~~~~~~~~~~~

Let's just add an ALREADY_CHECKED() statement there, no other check seems
to get rid of it. No backport is needed.