report-cgroup: use errno_or_else in one more place

report-cgroup: use errno_or_else in one more place

Old gcc is confused about initialization:
In function ‘io_read_send’,
    inlined from ‘walk_cgroups’ at ../src/report/report-cgroup.c:288:24:
../src/report/report-cgroup.c:167:21: error: ‘values[0]’ may be used uninitialized [-Werror=maybe-uninitialized]
  167 |                 r = metric_build_send_unsigned(mf + i, link, unit, values[i], /* fields= */ NULL);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Maybe this helps.