BUG/MINOR: stats: fix lastchk metric that got accidently lost

BUG/MINOR: stats: fix lastchk metric that got accidently lost

Commit d3a9a4992 ("MEDIUM: stats: allow to select one field in
`stats_fill_sv_stats`") left one occurrence of a direct assignment
of stats[] instead of placing it into the <metric> variable, and it
was on ST_F_CHECK_STATUS. This resulted in the field being overwritten
with an empty one immediately after being set in stats_fill_sv_stats()
and the field to appear empty on the stats page.

No backport is needed as this was only for 2.4.