Initialize search state best mode index

In av1_rd_pick_intermode_sb, the best mode index
is initialized appropriately to avoid possible issues
during winner mode processing.

BUG=aomedia:2429

Change-Id: Id6d83cd28de70af9d7bd00a82362c8c048e53e52
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index b7e4b63..461dd01 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -12868,6 +12868,7 @@
   if (do_tx_search_global != 1) {
     inter_modes_info_sort(inter_modes_info, inter_modes_info->rd_idx_pair_arr);
     search_state.best_rd = best_rd_so_far;
+    search_state.best_mode_index = -1;
 
     int64_t top_est_rd =
         inter_modes_info->num > 0