Fix valgrind error caused by NONE_FRAME in ref list

This error was reintroduced by
in an attempt to fix 91eb798b
a mismatch introduced by the original bug fix at
72e2e982.

BUG=aomedia:412

Change-Id: Iffc16fe0f5f3f953b0604a7cf56433350673338c

av1/decoder/decodemv.c

@@ -1697,16 +1697,14 @@ static void read_inter_block_mode_info(AV1Decoder *const pbi,
 #else
       zeromv[0].as_int = zeromv[1].as_int = 0;
 #endif
-      for (ref = 0; ref < 1 + is_compound; ++ref) {
+      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[ref]][0].as_int != zeromv[ref].as_int ||
+            ref_mvs[rf[ref]][1].as_int != zeromv[ref].as_int)
+          inter_mode_ctx[ref_frame] &= ~(1 << ALL_ZERO_FLAG_OFFSET);
       }
-
-      if (ref_mvs[rf[0]][0].as_int != zeromv[0].as_int ||
-          ref_mvs[rf[0]][1].as_int != zeromv[0].as_int ||
-          (is_compound && (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);
     }
   }