Initialize ref_list for simple motion search

Initialize it as LAST_FRAME to get around a case that ref_frame_flags
is not set up properly when the top level arf is not temporally
filtered. Need a better solution to fix the root cause.

Change-Id: Ibbb63c327de8f2b2be9186873e974f1bdb67e715

av1/encoder/partition_strategy.c

diff --git a/av1/encoder/partition_strategy.c b/av1/encoder/partition_strategy.c
index 221d95c..f2f8952 100644
--- a/av1/encoder/partition_strategy.c
+++ b/av1/encoder/partition_strategy.c
@@ -393,6 +393,7 @@
   assert(mi_size_wide[bsize] == mi_size_high[bsize]);
   // Setting up motion search
   int ref_list[1];
+  ref_list[0] = LAST_FRAME;
   if (!(cpi->ref_frame_flags & av1_ref_frame_flag_list[LAST_FRAME]) &&
       !(cpi->ref_frame_flags & av1_ref_frame_flag_list[ALTREF_FRAME])) {
     for (int i = LAST_FRAME; i <= ALTREF_FRAME; i++) {