dtcm: prevent to use NULL as a Tick type

Tick (aka time_t) shall be an integer type, not a pointer.
Change to use `0` instead of `NULL`.

cde/programs/dtcm/server/reclotick.c

@@ -81,12 +81,12 @@ ClosestTick(
 				 real_start_time,
 				 target_time = _target_time;
 
-	if (!re) return (Tick)NULL;
+	if (!re) return (Tick)0;
 
 	FillInRepeatEvent(start_time, re);
 
 	if (!(*res = InitRepeatEventState(re)))
-		return (Tick)NULL;
+		return (Tick)0;
 
 	if (target_time < start_time)
 		target_time = start_time;
@@ -131,7 +131,7 @@ ClosestTick(
 	    (re->re_end_date && re->re_end_date < closest_tick)) { 
 		free (*res);
 		*res = NULL;
-		return (Tick)NULL;
+		return (Tick)0;
 	}
 
 	/*
@@ -351,7 +351,7 @@ DoWeek(
 	/* Make sure the start time is on the first real event slot. */
 	if (_target_time) {
 		if (!(unused = InitRepeatEventState(re)))
-			return (Tick)NULL;
+			return (Tick)0;
 		start_time = DoWeek(0, _start_time, re, unused);
 		free(unused);
 		if (_target_time < start_time)
@@ -676,7 +676,7 @@ DoYearByMonth(
 	/* Make sure the start time is on the first real event slot. */
 	if (_target_time) {
 		if (!(unused = InitRepeatEventState(re)))
-			return (Tick)NULL;
+			return (Tick)0;
 		start_time = DoYearByMonth(0, _start_time, re, unused);
 		free(unused);
 		if (_target_time < start_time)
@@ -780,7 +780,7 @@ DoYearByDay(
 	/* Make sure the start time is on the first real event slot. */
 	if (_target_time) {
 		if (!(unused = InitRepeatEventState(re)))
-			return (Tick)NULL;
+			return (Tick)0;
 		start_time = DoYearByDay(0, _start_time, re, unused);
 		free(unused);
 		if (_target_time < start_time)

cde/programs/dtcm/server/relasttick.c

@@ -63,7 +63,7 @@ LastTick(
 {
 	Tick 			 last_time;
 
-	if (!re) return (Tick)NULL;
+	if (!re) return (Tick)0;
 
 	if (re->re_duration == RE_INFINITY) return EOT;
 
@@ -101,7 +101,7 @@ DoMinute(
 	const Tick		 start_time,
 	const RepeatEvent	*re)
 {
-	return (Tick)NULL;
+	return (Tick)0;
 }
 
 static Tick
@@ -404,7 +404,7 @@ DoYearByMonth(
 			}
 		}
 		/* No months have a day that can be used */
-		return ((Tick)NULL); 
+		return (Tick)0;
 	}
 
 	last_time2 = mktime(start_tm);

cde/programs/dtcm/server/renexttick.c

@@ -79,7 +79,7 @@ NextTick(
 {
 	Tick 			 next_time;
 
-	if (!re) return (Tick)NULL;
+	if (!re) return (Tick)0;
 
 	FillInRepeatEvent(start_time, re);
 
@@ -126,7 +126,7 @@ DoMinute(
 	const RepeatEvent	*re,
 	RepeatEventState	*res)
 {
-	return (Tick)NULL;
+	return (Tick)0;
 }
 
 static Tick

cde/programs/dtcm/server/reprevtick.c

@@ -92,7 +92,7 @@ PrevTick(
 	Tick 			 next_time;
 	Tick			 _start_time;
 
-        if (!re) return (Tick)NULL;
+        if (!re) return (Tick)0;
 
 	if (!start_time)
         	FillInRepeatEvent(cur_time, re);
@@ -148,7 +148,7 @@ DoMinute(
 	const RepeatEvent	*re,
 	RepeatEventState	*res)
 {
-	return (Tick)NULL;
+	return (Tick)0;
 }
 
 static Tick

cde/programs/dtcm/server/reutil.c

@@ -230,7 +230,7 @@ WeekNumberToDay(
          * month.
          */
         if (date_tm->tm_mon != initial_month_number)
-                return ((Tick)NULL);
+                return (Tick)0;
 
         return (_date);
 }