Call aom_clear_system_slate() with get_blk_sse_sum

Added aom_clear_system_slate() before double operation and afer aom_get_blk_sse_sum() is called in get_mean_dev_features.

Change-Id: I8ca4777f070864749c4d112f8dec5885ef64b058

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 0e8b90d..12a4a96 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -5023,6 +5023,7 @@
       total_x_sum += x_sum;
       total_x2_sum += x2_sum;
 
+      aom_clear_system_state();
       const float mean = (float)x_sum / sub_num;
       const float dev = get_dev(mean, (double)x2_sum, sub_num);
       feature[feature_idx++] = mean;