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.
diff --git a/src/stats.c b/src/stats.c
index 1218cb5..1f2a684 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -2299,7 +2299,7 @@
 					chunk_strcat(out, get_check_status_info(sv->check.status));
 					if (!(sv->check.state & CHK_ST_INPROGRESS))
 						fld_chksts += 2; // skip "* "
-					stats[ST_F_CHECK_STATUS] = mkf_str(FN_OUTPUT, fld_chksts);
+					metric = mkf_str(FN_OUTPUT, fld_chksts);
 				}
 				break;
 			case ST_F_CHECK_CODE: