roll_var: move prev_value out from cdef

Author: auderson

pandas/_libs/window/aggregations.pyx

@@ -362,7 +362,7 @@ def roll_var(const float64_t[:] values, ndarray[int64_t] start,
     """
     cdef:
         float64_t mean_x, ssqdm_x, nobs, compensation_add,
-        float64_t compensation_remove, prev_value = values[start[0]]
+        float64_t compensation_remove, prev_value
         int64_t s, e, num_consecutive_same_value = 0
         Py_ssize_t i, j, N = len(start)
         ndarray[float64_t] output
@@ -385,6 +385,8 @@ def roll_var(const float64_t[:] values, ndarray[int64_t] start,
             # never removed
             if i == 0 or not is_monotonic_increasing_bounds or s >= end[i - 1]:
 
+                prev_value = values[s]
+
                 mean_x = ssqdm_x = nobs = compensation_add = compensation_remove = 0
                 for j in range(s, e):
                     add_var(values[j], &nobs, &mean_x, &ssqdm_x, &compensation_add,