Disallow deleting towns which are referenced by a waypoint

See: #614

src/town_cmd.cpp

@@ -21,6 +21,7 @@
 #include "command_func.h"
 #include "industry.h"
 #include "station_base.h"
+#include "waypoint_base.h"
 #include "station_kdtree.h"
 #include "company_base.h"
 #include "news_func.h"
@@ -3463,6 +3464,11 @@ CommandCost CmdDeleteTown(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 		}
 	}
 
+	/* Waypoints refer to towns. */
+	for (const Waypoint *wp : Waypoint::Iterate()) {
+		if (wp->town == t) return CMD_ERROR;
+	}
+
 	/* Depots refer to towns. */
 	for (const Depot *d : Depot::Iterate()) {
 		if (d->town == t) return CMD_ERROR;