Avoid out-of-bounds issue 

When accessing to reference vector list with NONE_FRAME

BUG=aomedia:412

Change-Id: I82a23591d6d9a179eb6f3b1e40f8d1f4018a53d8

av1/decoder/decodemv.c

diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index a376948..5395a3c 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1696,15 +1696,14 @@
       zeromv[0].as_int = zeromv[1].as_int = 0;
 #endif
       for (ref = 0; ref < 2; ++ref) {
+        if (rf[ref] == NONE_FRAME) continue;
         lower_mv_precision(&ref_mvs[rf[ref]][0].as_mv, allow_hp);
         lower_mv_precision(&ref_mvs[rf[ref]][1].as_mv, allow_hp);
-      }
 
-      if (ref_mvs[rf[0]][0].as_int != zeromv[0].as_int ||
-          ref_mvs[rf[0]][1].as_int != zeromv[0].as_int ||
-          ref_mvs[rf[1]][0].as_int != zeromv[1].as_int ||
-          ref_mvs[rf[1]][1].as_int != zeromv[1].as_int)
-        inter_mode_ctx[ref_frame] &= ~(1 << ALL_ZERO_FLAG_OFFSET);
+        if (ref_mvs[rf[ref]][0].as_int != zeromv[0].as_int ||
+            ref_mvs[rf[ref]][1].as_int != zeromv[1].as_int)
+          inter_mode_ctx[ref_frame] &= ~(1 << ALL_ZERO_FLAG_OFFSET);
+      }
     }
   }