- added initialization of linked sectors

Resizing of linked sectors arrays during serialization could lead to processing of pointers to junk data

https://forum.zdoom.org/viewtopic.php?t=71252

src/gamedata/r_defs.h

@@ -522,6 +522,12 @@ struct FLinkedSector
 {
 	sector_t *Sector;
 	int Type;
+	
+	FLinkedSector(sector_t *sector = nullptr, int type = 0)
+	: Sector(sector)
+	, Type(type)
+	{
+	}
 };