Bugfix for ref1_counts index

In encode_superblock(), 'ref1_counts' was wrongly indexed based
on ref_frame[0]. Fixed this bug by indexing it based on ref_frame[1].

STATS_CHANGE expected

Change-Id: If1061c61828316cf1203d4996cfd3f3871725268
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index 6f1a296..f696245 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -6930,7 +6930,7 @@
         if (stats->ref0_counts[mbmi->ref_frame[0]] < 255)
           ++stats->ref0_counts[mbmi->ref_frame[0]];
         if (mbmi->ref_frame[1] >= 0 &&
-            stats->ref1_counts[mbmi->ref_frame[0]] < 255)
+            stats->ref1_counts[mbmi->ref_frame[1]] < 255)
           ++stats->ref1_counts[mbmi->ref_frame[1]];
       }
     }