fix roll_generic in window.pyx

Author: FXocena

pandas/_libs/window.pyx

@@ -1428,14 +1428,14 @@ def roll_generic(ndarray[float64_t, cast=True] input,
     if n == 0:
         return input
 
+    counts = roll_sum(np.concatenate([np.isfinite(input).astype(float),
+                                      np.array([0.] * offset)]),
+                      win, minp, index, closed)[offset:]
+    
     start, end, N, win, minp, is_variable = get_window_indexer(input, win,
                                                                minp, index,
                                                                closed,
                                                                floor=0)
-    
-    counts = roll_sum(np.concatenate([np.isfinite(input).astype(float),
-                                      np.array([0.] * offset)]),
-                      win, minp, index, closed)[offset:]
 
     output = np.empty(N, dtype=float)