(svn r3669) Really wipe all data when making a void tile

Thanks to Belugas, egladil and Rubidium for their inspiration and hard work, which all this stuff is based on

void.h

@@ -3,4 +3,11 @@
 static inline void MakeVoid(TileIndex t)
 {
 	SetTileType(t, MP_VOID);
+	SetTileHeight(t, 0);
+	_m[t].m1 = 0;
+	_m[t].m2 = 0;
+	_m[t].m3 = 0;
+	_m[t].m4 = 0;
+	_m[t].m5 = 0;
+	_m[t].extra = 0;
 }