Fix bug in interpolation_filter_rd function

Initialized RD_STATS structure with default value
to avoid valgrind issue.

BUG=aomedia:2427

Change-Id: I0deda968f5c306d7d808ac5330ab64c207bb6bb5

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 0c3451c..9184a46 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -8124,6 +8124,7 @@
   const AV1_COMMON *cm = &cpi->common;
   MACROBLOCKD *const xd = &x->e_mbd;
   RD_STATS tmp_rd_stats;
+  av1_init_rd_stats(&tmp_rd_stats);
 
   // Skip inter predictor if the predictor is already avilable.
   if (!is_skip_build_pred)