Fixes a potential crash with atcommand time (#4989)

* Fixes #4960.
* Adds a timer data validation check for atcommand time.
Thanks to @marky291!

src/map/atcommand.cpp

@@ -4851,18 +4851,22 @@ ACMD_FUNC(servertime)
 			clif_displaymessage(fd, msg_txt(sd,232)); // Game time: The game is in permanent night.
 	} else if (battle_config.night_duration == 0)
 		if (night_flag == 1) { // we start with night
-			timer_data = get_timer(day_timer_tid);
-			sprintf(temp, msg_txt(sd,233), txt_time(DIFF_TICK(timer_data->tick,gettick())/1000)); // Game time: The game is in night for %s.
-			clif_displaymessage(fd, temp);
-			clif_displaymessage(fd, msg_txt(sd,234)); // Game time: After, the game will be in permanent daylight.
+			if ((timer_data = get_timer(day_timer_tid)) != nullptr) {
+				sprintf(temp, msg_txt(sd,233), txt_time(DIFF_TICK(timer_data->tick,gettick())/1000)); // Game time: The game is in night for %s.
+				clif_displaymessage(fd, temp);
+				clif_displaymessage(fd, msg_txt(sd,234)); // Game time: After, the game will be in permanent daylight.
+			} else
+				clif_displaymessage(fd, msg_txt(sd,232)); // Game time: The game is in permanent night.
 		} else
 			clif_displaymessage(fd, msg_txt(sd,231)); // Game time: The game is in permanent daylight.
 	else if (battle_config.day_duration == 0)
 		if (night_flag == 0) { // we start with day
-			timer_data = get_timer(night_timer_tid);
-			sprintf(temp, msg_txt(sd,235), txt_time(DIFF_TICK(timer_data->tick,gettick())/1000)); // Game time: The game is in daylight for %s.
-			clif_displaymessage(fd, temp);
-			clif_displaymessage(fd, msg_txt(sd,236)); // Game time: After, the game will be in permanent night.
+			if ((timer_data = get_timer(night_timer_tid)) != nullptr) {
+				sprintf(temp, msg_txt(sd,235), txt_time(DIFF_TICK(timer_data->tick,gettick())/1000)); // Game time: The game is in daylight for %s.
+				clif_displaymessage(fd, temp);
+				clif_displaymessage(fd, msg_txt(sd,236)); // Game time: After, the game will be in permanent night.
+			} else
+				clif_displaymessage(fd, msg_txt(sd,231)); // Game time: The game is in permanent daylight.
 		} else
 			clif_displaymessage(fd, msg_txt(sd,232)); // Game time: The game is in permanent night.
 	else {