BUILD: config: fix again bugs gcc warnings on calloc

BUILD: config: fix again bugs gcc warnings on calloc

Since commit ad37c7ab ("BUILD: config: address build warning on
raspbian+rpi4") gcc 7.3.0 complains again on x86_64 (while 8.2.0
does not) :

  src/cfgparse.c: In function 'check_config_validity':
  src/cfgparse.c:3593:26: warning: argument 1 range [18446744071562067968, 18446744073709551615] exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]
       newsrv->idle_conns = calloc(global.nbthread, sizeof(*newsrv->idle_conns));
                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This thing is completely bogus (actually the RPi one was the most wrong).
Let's try to shut them both by using an unsigned short for the cast which
is expected to satisfy everyone. It's worth noting that the exact same call
a few lines above and below do not trigger this stupid warning.

This should be backported to 2.2 since the fix above was put there already.