MINOR: stats: properly handle ST_F_CHECK_DURATION metric

MINOR: stats: properly handle ST_F_CHECK_DURATION metric

ST_F_CHECK_DURATION metric is typed as unsigned int variable, and it is
derived from check->duration that is signed.

While most of the time check->duration > 0, it is not always true:
with HCHK_STATUS_HANA checks, check->duration is set to -1 to prevent server
logs from including irrelevant duration info (HCHK_STATUS_HANA checks are not
time related).

Because of this, stats could report UINT64_MAX value for ST_F_CHECK_DURATION
metric. This was quite confusing. To prevent this, we make sure not to assign
negative value to ST_F_CHECK_DURATION.

This is only a minor printing issue, not backport needed.