examples/moveplayer: default dx and dy to 0 for the player

examples/gameplay/moveplayer/actors/player.lua

@@ -7,7 +7,7 @@ function Player:new(world, x, y)
 end
 
 function Player:update(dt)
-  local dx, dy
+  local dx, dy = 0, 0
   if self.keys["up"].isPressed then
     dy = -120 * dt
   end