fix xNES telling

xnes.lua

@@ -179,12 +179,12 @@ function Xnes:tell(scored, noise)
 
     -- finally, update according to the gradients.
 
+    local dotted = dot_mv(self.covars, g_delta)
     local step = {}
     for i, v in ipairs(dotted) do
         step[i] = self.sigma * v
     end
 
-    local dotted = dot_mv(self.covars, g_delta)
     for i, v in ipairs(self.mean) do
         self.mean[i] = v + self.mean_adapt * step[i]
     end