fixes [core-tcl/482db1d796540e68]: local seconds of day shall be always positive, also for negative base (modulo operation may be negative in C)

Author: sebres

generic/tclClock.c

@@ -4441,7 +4441,11 @@ ClockAddObjCmd(
     }
 
     /* time together as seconds of the day */
-    yySecondOfDay = yySeconds = yydate.localSeconds % SECONDS_PER_DAY;
+    yySecondOfDay = yydate.localSeconds % SECONDS_PER_DAY;
+    if (yySecondOfDay < 0) { /* to positive result of mod */
+	yySecondOfDay += SECONDS_PER_DAY;
+    }
+    yySeconds = yySecondOfDay;
     /* seconds are in localSeconds (relative base date), so reset time here */
     yyHour = 0; yyMinutes = 0; yyMeridian = MER24;