Fix a bug in deciding ref frame context in ext-refs

Change-Id: Ie58b98baa870c5d2a5b7193f8fe4f84fd7ec6c16

vp10/common/pred_common.c

diff --git a/vp10/common/pred_common.c b/vp10/common/pred_common.c
index dc0cd90..27819b9 100644
--- a/vp10/common/pred_common.c
+++ b/vp10/common/pred_common.c
@@ -600,7 +600,7 @@
       const MB_MODE_INFO *edge_mbmi = above_intra ? left_mbmi : above_mbmi;
 
       if (!has_second_ref(edge_mbmi))  // single pred (1/3)
-        pred_context = 1 + 2 * (edge_mbmi->ref_frame[1] != cm->comp_bwd_ref[1]);
+        pred_context = 1 + 2 * (edge_mbmi->ref_frame[0] != cm->comp_bwd_ref[1]);
       else  // comp pred (1/3)
         pred_context =
             1 +