Add a call to aom_clear_system_state()

Added aom_clear_system_state() in predict_dc_only_block
function before double operation.

BUG=aomedia:2826

Change-Id: Ic6774c410390ebbd3abe4dab506626d7f4847437

av1/encoder/tx_search.c

diff --git a/av1/encoder/tx_search.c b/av1/encoder/tx_search.c
index 82100d1..0b2ca47 100644
--- a/av1/encoder/tx_search.c
+++ b/av1/encoder/tx_search.c
@@ -419,6 +419,7 @@
   int sum = 0;
   sse = aom_sum_sse_2d_i16(diff, diff_stride, visible_cols, visible_rows, &sum);
   if (visible_cols > 0 && visible_rows > 0) {
+    aom_clear_system_state();
     double norm_factor = 1.0 / (visible_cols * visible_rows);
     int sign_sum = sum > 0 ? 1 : -1;
     // Conversion to transform domain