Add sanity check on ref frame index

This is to fix the decoder crash on reading the is_inter_block flag
for the segment active scenario, when the segment is intra coded.

BUG=aomedia:1722

Change-Id: I25bb71df849e6fe8347a8cbc143566df4fbcfe83

av1/decoder/decodemv.c

diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index 3326be0..531926d 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1224,6 +1224,7 @@
                                int segment_id, aom_reader *r) {
   if (segfeature_active(&cm->seg, segment_id, SEG_LVL_REF_FRAME)) {
     const int frame = get_segdata(&cm->seg, segment_id, SEG_LVL_REF_FRAME);
+    if (frame < LAST_FRAME) return 0;
     RefBuffer *ref_buf = &cm->frame_refs[frame - LAST_FRAME];
     return frame != INTRA_FRAME && av1_is_valid_scale(&ref_buf->sf);
   }