Prevent invaid memory access

Reading second motion vector only when it has a second ref_frame

Change-Id: Ica72c1cd955832e15ceccda5e5a17b0bfcd83044

vp9/encoder/vp9_encodeframe.c

diff --git a/vp9/encoder/vp9_encodeframe.c b/vp9/encoder/vp9_encodeframe.c
index 9966cb6..9d02c8f 100644
--- a/vp9/encoder/vp9_encodeframe.c
+++ b/vp9/encoder/vp9_encodeframe.c
@@ -500,7 +500,7 @@
     if (is_inter_block(mbmi) &&
         (mbmi->sb_type < BLOCK_8X8 || mbmi->mode == NEWMV)) {
       int_mv best_mv[2];
-      for (i = 0; i < 2; ++i)
+      for (i = 0; i < 1 + has_second_ref(mbmi); ++i)
         best_mv[i].as_int = mbmi->ref_mvs[mbmi->ref_frame[i]][0].as_int;
       vp9_update_mv_count(cpi, x, best_mv);
     }