Codechange: Silence -Wclass-memaccess warnings with GCC8

src/landscape.cpp

@@ -1080,8 +1080,7 @@ static uint River_Hash(uint tile, uint dir)
  */
 static void BuildRiver(TileIndex begin, TileIndex end)
 {
-	AyStar finder;
-	MemSetT(&finder, 0);
+	AyStar finder = {};
 	finder.CalculateG = River_CalculateG;
 	finder.CalculateH = River_CalculateH;
 	finder.GetNeighbours = River_GetNeighbours;